Testing subterranean water for a hazardous waste material repository

ABSTRACT

Techniques for determining the suitability of a subterranean formation as a hazardous waste repository include determining a neutron flux of a first isotope in a subterranean formation; calculating, based at least in part on the determined neutron flux of the first isotope, a predicted production rate of a second isotope in the subterranean formation; calculating a first ratio of the predicted production rate of the second isotope relative to a theoretical production rate of a stable form of the second isotope; measuring respective concentrations of the second isotope and the stable form of the second isotope in a subterranean water sample; calculating a second ratio of the measured concentration of the second isotope relative to the measured concentration of the stable form of the second isotope; and based on a comparison of the first and second ratios, determining that the subterranean formation is suitable as a hazardous waste repository.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 16/796,798, filed on Feb. 20, 2020, and entitled“TESTING SUBTERRANEAN WATER FOR A HAZARDOUS WASTE MATERIAL REPOSITORY,”which in turn claims priority under 35 U. S.C. § 119 to: U.S.Provisional Patent Application Ser. No. 62/808,523, filed on Feb. 21,2019; U.S. Provisional Patent Application Ser. No. 62/833,285, filed onApr. 12, 2019; U.S. Provisional Patent Application Ser. No. 62/911,560,filed on Oct. 7, 2019; and U.S. Provisional Patent Application Ser. No.62/934,894, filed on Nov. 13, 2019. The entire contents of each of theprevious applications are incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to testing subterranean water and, moreparticular, testing subterranean water for one or more radioactiveisotopes for a hazardous waste material repository.

BACKGROUND

Storing hazardous waste material underground may have significant risks.One risk may be that the hazardous waste material, or byproducts of thehazardous waste material, may enter into a source of human-consumablewater. Some subterranean formations allow mobile water; that is themovement of water stored in the formation to a location in whichhuman-consumable water is located. Therefore, any hazardous wastematerial stored underground must be kept from a source of mobile water.

SUMMARY

In a general implementation, a method includes determining aconcentration of at least one noble gas isotope of a plurality of noblegas isotopes in fluid sample from a subterranean formation below aterranean surface; determining a produced amount of the at least onenoble gas isotope in the subterranean formation based on a productionrate of the at least one noble gas isotope and a minimum residence time;calculating a ratio of the determined concentration of the at least onenoble gas isotope in the fluid sample to the determined produced amountof the at least one noble gas isotope; and based on the calculated ratiobeing at or near a threshold value, determining that the subterraneanformation is suitable as a hazardous waste repository.

In an aspect combinable with the general implementation, the productionrate is based on one or more properties of rock in the subterraneanformation.

In another aspect combinable with any of the previous aspects, the oneor more properties include a bulk rock chemistry of the subterraneanformation.

In another aspect combinable with any of the previous aspects, the oneor more properties include an amount of uranium or thorium, or both, perunit volume in the subterranean formation.

In another aspect combinable with any of the previous aspects, the oneor more properties include a decay rate of uranium or thorium, or both,in the subterranean formation.

In another aspect combinable with any of the previous aspects, the atleast one noble gas isotope includes at least one of helium (He), neon(Ne), argon (Ar), krypton (Kr), or xenon (Xe).

Another aspect combinable with any of the previous aspects furtherincludes determining a concentration of another noble gas isotope of theplurality of noble gas isotopes in a fluid sample from a subterraneanformation below a terranean surface; determining a produced amount ofthe another noble gas isotope in the subterranean formation based on aproduction rate of the another noble gas isotope and a minimum residencetime that is sufficient to show that the subterranean formation issuitable as a hazardous waste repository; calculating another ratio ofthe determined concentration of the another noble gas isotope in thewater sample to the determined produced amount of the another noble gasisotope; and based on the another calculated ratio being at or near athreshold value, determining that the subterranean formation is suitableas the hazardous waste repository.

Another aspect combinable with any of the previous aspects furtherincludes comparing the calculated ratio with the another calculatedratio; and based on the comparison, determining that the subterraneanformation is suitable as the hazardous waste repository.

In another aspect combinable with any of the previous aspects, the atleast one noble gas isotope is produced from a first production mode,and the another noble gas isotope is produced from a second productionmode different than the first production mode.

Another aspect combinable with any of the previous aspects furtherincludes collecting the water sample from a drillhole formed into thesubterranean formation.

In another aspect combinable with any of the previous aspects,collecting the water sample includes operating a downhole tool in thedrillhole to collect a core sample from the subterranean formation;retrieving the core sample to the terranean surface; and removing thewater sample from the core sample.

Another aspect combinable with any of the previous aspects furtherincludes forming the drillhole from the terranean surface to thesubterranean formation.

In another aspect combinable with any of the previous aspects, thedrillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a shale formation.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes an impermeable layer.

In another aspect combinable with any of the previous aspects, theminimum residence time is at least 10,000.

In another aspect combinable with any of the previous aspects, thethreshold value is one.

Another aspect combinable with any of the previous aspects furtherincludes initiating creation of the hazardous waste repository in orunder the subterranean formation.

In another aspect combinable with any of the previous aspects,initiating creation of the hazardous waste repository in or under thesubterranean formation includes forming an access drillhole from theterranean surface toward the subterranean formation; and forming astorage drillhole coupled to the access drillhole in or under thesubterranean formation, the storage drillhole including a hazardouswaste storage area.

In another aspect combinable with any of the previous aspects, theaccess drillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thedrillhole includes a portion of the access drillhole.

In another aspect combinable with any of the previous aspects, thestorage drillhole includes a curved portion and a horizontal portion.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area of at least about 200 feet.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area that inhibits diffusion of a hazardous waste materialthrough the subterranean formation for an amount of time that is basedon a half-life of the hazardous waste material.

Another aspect combinable with any of the previous aspects furtherincludes installing a casing in the access drillhole and the storagedrillhole that extends from at or proximate the terranean surface,through the access drillhole and the storage drillhole, and into thehazardous waste storage area of the storage drillhole.

Another aspect combinable with any of the previous aspects furtherincludes cementing the casing to the access drillhole and the storagedrillhole.

Another aspect combinable with any of the previous aspects furtherincludes, subsequent to forming the access drillhole, producinghydrocarbon fluid from the subterranean formation, through the accessdrillhole, and to the terranean surface.

Another aspect combinable with any of the previous aspects furtherincludes storing hazardous waste material in the hazardous waste storagearea.

In another aspect combinable with any of the previous aspects, storinghazardous waste material in the hazardous waste storage area includesmoving a storage canister through an entry of the access drillhole thatextends into the terranean surface, the entry at least proximate theterranean surface, the storage canister including an inner cavity sizedto enclose the hazardous waste material; moving the storage canisterthrough the access drillhole and into the storage drillhole; and movingthe storage canister through the storage drillhole to the hazardouswaste storage area.

Another aspect combinable with any of the previous aspects furtherincludes forming a seal in at least one of the access drillhole or thestorage drillhole that isolates the hazardous waste storage area fromthe entry of the access drillhole.

In another aspect combinable with any of the previous aspects, thehazardous waste material includes spent nuclear fuel.

In another aspect combinable with any of the previous aspects, thestorage canister includes a connecting portion configured to couple toat least one of a downhole tool string or another storage canister.

Another aspect combinable with any of the previous aspects furtherincludes monitoring the hazardous waste material stored in the hazardouswaste storage area.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area includes removing the seal; and retrieving the storagecanister from the hazardous waste storage area to the terranean surface.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area of the storage drillhole includes monitoring at least onevariable associated with the storage canister from a sensor positionedproximate the hazardous waste storage area; and recording the monitoredvariable at the terranean surface.

In another aspect combinable with any of the previous aspects, themonitored variable includes at least one of radiation level,temperature, pressure, presence of oxygen, presence of water vapor,presence of liquid water, acidity, or seismic activity.

Another aspect combinable with any of the previous aspects furtherincludes, based on the monitored variable exceeding a threshold value,removing the seal; and retrieving the storage canister from thehazardous waste storage area to the terranean surface.

In another aspect combinable with any of the previous aspects, the fluidsample includes liquid brine.

In another aspect combinable with any of the previous aspects, theminimum residence time is sufficient to show that the subterraneanformation is suitable as the hazardous waste repository.

In another general implementation, a method includes calculating aconcentration of a krypton isotope in a subterranean water samplecollected from a subterranean formation; determining that theconcentration of the krypton isotope is less than a threshold value; andbased on the determination, determining that the subterranean formationis suitable as a hazardous waste repository.

In an aspect combinable with the general implementation, the kryptonisotope is Kr-81.

In another aspect combinable with any of the previous aspects,calculating includes using Atom Trap Trace Analysis to calculate theconcentration of the krypton isotope.

In another aspect combinable with any of the previous aspects, thethreshold value is based on a concentration of the krypton isotope in asurface water sample.

In another aspect combinable with any of the previous aspects, thethreshold value is between 16-20 atoms of the krypton isotope in thesubterranean water sample.

Another aspect combinable with any of the previous aspects furtherincludes collecting the subterranean water sample from a drillholeformed into the subterranean formation.

In another aspect combinable with any of the previous aspects,collecting the subterranean water sample includes operating a downholetool in the drillhole to collect a core sample from the subterraneanformation; retrieving the core sample to the terranean surface; andremoving the subterranean water sample from the core sample.

Another aspect combinable with any of the previous aspects furtherincludes forming the drillhole from the terranean surface to thesubterranean formation.

In another aspect combinable with any of the previous aspects, thedrillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a shale formation.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes an impermeable layer.

Another aspect combinable with any of the previous aspects furtherincludes initiating creation of the hazardous waste repository in orunder the subterranean formation.

In another aspect combinable with any of the previous aspects,initiating creation of the hazardous waste repository in or under thesubterranean formation includes forming an access drillhole from theterranean surface toward the subterranean formation; and forming astorage drillhole coupled to the access drillhole in or under thesubterranean formation, the storage drillhole including a hazardouswaste storage area.

In another aspect combinable with any of the previous aspects, theaccess drillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thedrillhole includes a portion of the access drillhole.

In another aspect combinable with any of the previous aspects, thestorage drillhole includes a curved portion and a horizontal portion.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area of at least about 200 feet.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area that inhibits diffusion of a hazardous waste materialthrough the subterranean formation for an amount of time that is basedon a half-life of the hazardous waste material.

Another aspect combinable with any of the previous aspects furtherincludes installing a casing in the access drillhole and the storagedrillhole that extends from at or proximate the terranean surface,through the access drillhole and the storage drillhole, and into thehazardous waste storage area of the storage drillhole.

Another aspect combinable with any of the previous aspects furtherincludes cementing the casing to the access drillhole and the storagedrillhole.

Another aspect combinable with any of the previous aspects furtherincludes, subsequent to forming the access drillhole, producinghydrocarbon fluid from the subterranean formation, through the accessdrillhole, and to the terranean surface.

Another aspect combinable with any of the previous aspects furtherincludes storing hazardous waste material in the hazardous waste storagearea.

In another aspect combinable with any of the previous aspects, storinghazardous waste material in the hazardous waste storage area includesmoving a storage canister through an entry of the access drillhole thatextends into the terranean surface, the entry at least proximate theterranean surface, the storage canister including an inner cavity sizedto enclose the hazardous waste material; moving the storage canisterthrough the access drillhole and into the storage drillhole; and movingthe storage canister through the storage drillhole to the hazardouswaste storage area.

Another aspect combinable with any of the previous aspects furtherincludes forming a seal in at least one of the access drillhole or thestorage drillhole that isolates the hazardous waste storage area fromthe entry of the access drillhole.

In another aspect combinable with any of the previous aspects, thehazardous waste material includes spent nuclear fuel.

In another aspect combinable with any of the previous aspects, thestorage canister includes a connecting portion configured to couple toat least one of a downhole tool string or another storage canister.

Another aspect combinable with any of the previous aspects furtherincludes monitoring the hazardous waste material stored in the hazardouswaste storage area.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area includes removing the seal; and retrieving the storagecanister from the hazardous waste storage area to the terranean surface.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area of the storage drillhole includes monitoring at least onevariable associated with the storage canister from a sensor positionedproximate the hazardous waste storage area; and recording the monitoredvariable at the terranean surface.

In another aspect combinable with any of the previous aspects, themonitored variable includes at least one of radiation level,temperature, pressure, presence of oxygen, presence of water vapor,presence of liquid water, acidity, or seismic activity.

Another aspect combinable with any of the previous aspects furtherincludes, based on the monitored variable exceeding a threshold value,removing the seal; and retrieving the storage canister from thehazardous waste storage area to the terranean surface.

In another aspect combinable with any of the previous aspects, thesubterranean water sample includes brine.

In another general implementation, a method includes determining aneutron flux of a first isotope in a subterranean formation;calculating, based at least in part on the determined neutron flux ofthe first isotope, a predicted production rate of a second isotope inthe subterranean formation; calculating a first ratio of the predictedproduction rate of the second isotope relative to a theoreticalproduction rate of a stable form of the second isotope; measuringrespective concentrations of the second isotope and the stable form ofthe second isotope in a subterranean water sample from the subterraneanformation; calculating a second ratio of the measured concentration ofthe second isotope relative to the measured concentration of the stableform of the second isotope; and based on a comparison of the first andsecond ratios, determining that the subterranean formation is suitableas a hazardous waste repository.

In an aspect combinable with the general implementation, the firstisotope includes a first half-life, and the second isotope includes asecond half-life longer than the first half-life.

In another aspect combinable with any of the previous aspects, the firstisotope includes Ar-39, Fe-59, Co-60, Ni-63, Kr-85, Ni-63, or C-14.

In another aspect combinable with any of the previous aspects, thesecond isotope includes Cl-36.

In another aspect combinable with any of the previous aspects, thestable form of the second isotope includes Cl-35.

In another aspect combinable with any of the previous aspects, comparingthe first and second ratios includes determining that the first andsecond ratios are equal.

In another aspect combinable with any of the previous aspects,determining the neutron flux of the first isotope includes determiningthe neutron flux based on a bulk rock chemistry of the subterraneanformation.

In another aspect combinable with any of the previous aspects, the bulkrock chemistry includes an amount of uranium or thorium (or both) perunit volume of the subterranean formation.

Another aspect combinable with any of the previous aspects furtherincludes collecting the subterranean water sample from a drillholeformed into the subterranean formation.

In another aspect combinable with any of the previous aspects,collecting the subterranean water sample includes operating a downholetool in the drillhole to collect a core sample from the subterraneanformation; retrieving the core sample to the terranean surface; andremoving the subterranean water sample from the core sample.

Another aspect combinable with any of the previous aspects furtherincludes forming the drillhole from the terranean surface to thesubterranean formation.

In another aspect combinable with any of the previous aspects, thedrillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a shale formation.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes an impermeable layer.

Another aspect combinable with any of the previous aspects furtherincludes initiating creation of the hazardous waste repository in orunder the subterranean formation.

In another aspect combinable with any of the previous aspects,initiating creation of the hazardous waste repository in or under thesubterranean formation includes forming an access drillhole from theterranean surface toward the subterranean formation; and forming astorage drillhole coupled to the access drillhole in or under thesubterranean formation, the storage drillhole including a hazardouswaste storage area.

In another aspect combinable with any of the previous aspects, theaccess drillhole includes a vertical drillhole.

In another aspect combinable with any of the previous aspects, thedrillhole includes a portion of the access drillhole.

In another aspect combinable with any of the previous aspects, thestorage drillhole includes a curved portion and a horizontal portion.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area of at least about 200 feet.

In another aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste storage area that inhibits diffusion of a hazardous waste materialthrough the subterranean formation for an amount of time that is basedon a half-life of the hazardous waste material.

Another aspect combinable with any of the previous aspects furtherincludes installing a casing in the access drillhole and the storagedrillhole that extends from at or proximate the terranean surface,through the access drillhole and the storage drillhole, and into thehazardous waste storage area of the storage drillhole.

Another aspect combinable with any of the previous aspects furtherincludes cementing the casing to the access drillhole and the storagedrillhole.

Another aspect combinable with any of the previous aspects furtherincludes, subsequent to forming the access drillhole, producinghydrocarbon fluid from the subterranean formation, through the accessdrillhole, and to the terranean surface.

Another aspect combinable with any of the previous aspects furtherincludes storing hazardous waste material in the hazardous waste storagearea.

In another aspect combinable with any of the previous aspects, storinghazardous waste material in the hazardous waste storage area includesmoving a storage canister through an entry of the access drillhole thatextends into the terranean surface, the entry at least proximate theterranean surface, the storage canister including an inner cavity sizedto enclose the hazardous waste material; moving the storage canisterthrough the access drillhole and into the storage drillhole; and movingthe storage canister through the storage drillhole to the hazardouswaste storage area.

Another aspect combinable with any of the previous aspects furtherincludes forming a seal in at least one of the access drillhole or thestorage drillhole that isolates the hazardous waste storage area fromthe entry of the access drillhole.

In another aspect combinable with any of the previous aspects, thehazardous waste material includes spent nuclear fuel.

In another aspect combinable with any of the previous aspects, thestorage canister includes a connecting portion configured to couple toat least one of a downhole tool string or another storage canister.

Another aspect combinable with any of the previous aspects furtherincludes monitoring the hazardous waste material stored in the hazardouswaste storage area.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area includes removing the seal; and retrieving the storagecanister from the hazardous waste storage area to the terranean surface.

In another aspect combinable with any of the previous aspects,monitoring the hazardous waste material stored in the hazardous wastestorage area of the storage drillhole includes monitoring at least onevariable associated with the storage canister from a sensor positionedproximate the hazardous waste storage area; and recording the monitoredvariable at the terranean surface.

In another aspect combinable with any of the previous aspects, themonitored variable includes at least one of radiation level,temperature, pressure, presence of oxygen, presence of water vapor,presence of liquid water, acidity, or seismic activity.

Another aspect combinable with any of the previous aspects furtherincludes, based on the monitored variable exceeding a threshold value,removing the seal; and retrieving the storage canister from thehazardous waste storage area to the terranean surface.

Implementations of subterranean water testing systems and methodsaccording to the present disclosure may also include one or more of thefollowing features. For example, subterranean water testing systems andmethods according to the present disclosure may be used to identify ordetermine that a particular subterranean formation is suitable as ahazardous waste material repository. The determined hazardous wastematerial repository may be used to store hazardous waste material, suchas spent nuclear fuel, isolated from human-consumable water sources. Thedetermined hazardous waste material repository may be suitable forstoring the hazardous waste material for durations of time up to, forexample, 1,000,000 years. As another example, subterranean water testingsystems and methods according to the present disclosure may confirm thata particular geologic formation is suitable as a hazardous wastematerial repository.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example implementation of asubterranean water testing system according to the present disclosure.

FIG. 2A is a schematic illustration of an example implementation of ahazardous waste material storage repository system during a deposit orretrieval operation according to the present disclosure.

FIG. 2B is a schematic illustration of an example implementation of ahazardous waste material storage repository system during storage ofhazardous waste material.

FIGS. 3A-3F are flowcharts that illustrate example processes for testingsubterranean water to determine suitability of a subterranean formationas a hazardous waste repository according to the present disclosure.

FIG. 4 is a flowchart that illustrates an example process for storinghazardous waste material in a subterranean formation from which waterhas been tested for a radioactive isotope concentration percentage.

FIG. 5 is a schematic illustration of a controller or control systemaccording to the present disclosure.

DETAILED DESCRIPTION

Radioactive waste (e.g., spent nuclear fuel, high level waste, orotherwise) includes components that last for thousands to hundreds ofthousands of years. The radioactive waste can be disposed of in deepdirectional drillholes (e.g., human-unoccupiable drillholes or wellboresthat are too small for a human to enter or occupy) that form or comprisea hazardous waste repository (also human-unoccupiable) that is formed ina subterranean formation. For example, high-level radioactive waste andother hazardous materials can be disposed in horizontal drillholes. Whenso disposed, a threat to human safety is that some of the radioactivewaste can be dissolved or otherwise incorporated into brines and otherwater that is often found at the same depth as the stored radioactivewaste. The flow of these liquids could carry the radioactive waste to aterranean surface or to aquifers (or other water sources). Because thetoxicity of the radioactive waste is long lived, it is desirable thatthe subterranean formation in which the radioactive waste is stored showthat fluids in such formation be demonstrably isolated (e.g., from theterranean surface or water sources) for thousands, to tens of thousands,to hundreds of thousands, to a million years or more.

In some aspects, isolation may be demonstrated by showing that any flowof liquids in such a subterranean formation be extremely slow. Forexample, if the radioactive waste is disposed at a depth of 1000 meters(m), then a flow of liquid in the formation of 1 millimeter (mm) peryear could bring such waste (e.g., dissolved or entrained in the liquid)to the surface in a million years. Current measuring techniques areunable of making direct determination of such tiny flow velocities. As aresult, safety for underground disposal is typically demonstrated bymeasurement of the permeability of the rock at depth, combined withplausible assumptions of driving forces (e.g., pressure gradients in thefluid) to calculate, often with complex computer code, the expected rateof flow of deep fluids to the surface.

An alternative or supplementary technique to estimate isolation can bebased on the past history of the rock formation that holds therepository. If the brines or other liquids in the formation can be shownto have persisted in the subterranean formation for, e.g., a millionyears, without flow toward the terranean surface, then such persistenceis evidence of the isolative properties of the subterranean formation,making it a possible location for a hazardous waste repository. Such adetermination is based, for example, on the fact that unlike surfacefeatures, underground geology changes extremely slowly. Events that aredifficult to model, such as creation of new earthquake faults, can beassumed to be no more prevalent in the future than they have been in thepast.

FIG. 1 is a schematic illustration of an example implementation of asubterranean water testing system 100. As shown in this example, thesystem 100 includes a test drillhole 104 formed from a terranean surface102, through a surface water formation 106, and into and throughsubterranean formations 108 and 110 that are deeper than the surfacewater formation 106. Each of the formations 106, 108, and 110 maycomprise a geologic formation formed of one or more rock types, as wellas water (e.g., fresh or brine) and in some cases other fluids (e.g.,hydrocarbon fluids). In this example, the test drillhole 104 is shown asa vertical drillhole. However, in alternative implementations, adirectional drillhole 124 (shown in dashed line) may be formed and usedin the system 100 in place of (or in addition to) the test drillhole104) according to the present disclosure.

The test drillhole 104 may be drilled or otherwise formed from theterranean formation to one or both of the subterranean formations 108 or110 (as well as formations shallower than or deeper than one or both ofthe formations 108 or 110). Test drillhole 104, may be relativelysmaller (e.g., in diameter) than a wellbore formed for the purpose ofproducing hydrocarbons. Alternatively, test drillhole 104 may be similarto a wellbore formed for the purpose of producing hydrocarbon and, insome aspects, may have had hydrocarbons produced therefrom.

The surface water formation 106, in this example, is a geologic layercomprised of one or more layered rock formations and includes one ormore surface water sources. For example, surface water formation 106 mayinclude one or more open water sources 116 (e.g., lakes, ponds, rivers,creeks). In some aspects, open water sources 116 are water sources thathave direct contact with the atmosphere 101. Surface water formation 106may also include one or more aquifers 118 that are not in direct contactwith the atmosphere 101 but are suitable for human consumption (e.g.,with or without conventional water treatment). Thus, in this exampleimplementation of system 100, surface water includes both open watersources 116 and aquifers 118. Examples of rock formations of which thesurface water formation 106 may be composed include porous sandstonesand limestones, among other formations.

Below the surface water formation 106, in this example implementation,are subterranean formations 108 and 110. One or both of the subterraneanformations 108 or 110 may include or hold subterranean water.Subterranean water, in this example system, is water that is not an openwater source or aquifer and is not in present-day contact with theatmosphere 101 (but may have been at some time in the past). In someaspects, subterranean water is non-potable or is not fit for humanconsumption (or both). System 100 may be used (e.g., as described withreference to FIG. 3 and process 300) to test one or both of subterraneanformations 108 or 110 for hazardous waste material storage according tothe subterranean water found in such formations.

System 100 also includes a downhole tool 112 (e.g., a core drill) thatcan be conveyed into the test drillhole 104 and to one or all offormations 106, 108, and 110 to procure a core sample 114 or core sample120. In this example, core sample 114 include subterranean water whilecore sample 120 includes surface water. Thus, a subterranean watersample may be obtained from core sample 114, while a surface watersample may be obtained from core sample 120 (or open water source 116 oraquifer 118). Although core sample 114 is shown as being obtained fromsubterranean formation 110, one or more core samples 114 may be obtainedfrom this formation or subterranean formation 108 (or both).

System 100 also includes a subterranean water (or other fluid) testingsystem, represented schematically as testing system 122. As described inmore detail herein, the testing system 122 may perform one or more testson, e.g., a subterranean fluid (e.g., water such as brine) sample and,in some aspects, a surface fluid (e.g., water or other liquid) sample.As described more specifically herein, the testing system 122 may be, insome aspects, a processor-based control system that testes thesubterranean fluid sample (and in some aspects, the surface fluidsample) for the presence, or concentration, of one or more radioisotopes(stable or otherwise) within the sample(s).

In an example implementation, the fluid testing system 122 includes anaccelerator mass spectrometry system (AMS). The AMS system, generally,may be operated to perform many testing functions. For example, the AMSsystem may analyze substances, such as water, to detect naturallyoccurring, long-lived radio-isotopes (of elements) such as beryllium-10(¹⁰Be), chlorine-36 (³⁶Cl), aluminum-26 (²⁶Al), iodine-129 (¹²⁹I) andcarbon-14 (i.e., radiocarbon or ¹⁴C) in such substances. In some cases,certain radioactive isotopes, such as ³⁶Cl and ¹²⁹I, may be produced inthe atmosphere 101 by cosmic radiation, and mixed with surface water, oris produced directly in the surface water or surface rock. Thus,substances such as surface water sources may have a particularconcentration of such radioactive isotopes of the elements based on timeperiod of the atmosphere 101 to which the substances have been exposed.Substances no longer exposed to the atmosphere 101, such as subterraneanwater, experience a decay in the concentration of such radioactiveisotopes (e.g., ³⁶Cl relative to the concentration of the stableisotope, ³⁷Cl, of the element chlorine; ¹²⁹I relative to theconcentration of the stable isotope, ¹²⁷I, of the element iodine; ¹⁰Berelative to the concentration of the stable isotope, ⁹Be, of the elementberyllium; ¹⁴C relative to the concentration of the stable isotopes, ¹²Cor ¹³C, of the element carbon; ²⁶Al relative to the concentration of thestable isotope, ²⁷Al, of the element aluminum) as time passes withoutsuch exposure. Thus, a measure of a concentration of radioactiveisotopes in a substance, such as subterranean water, may also indicatean amount of time that has passed since the substance was last exposedto the atmosphere 101 or surface water.

In an example operation of the system 100 including an AMS as the fluidtesting system 122 (or part of the system 122), a subterranean watersample may be collected from the drillhole 104 (or from a core samplefrom one or both of the subterranean formations 108 and 110).“Collecting” may include or mean identifying a previously gatheredsubterranean water sample. A surface water sample may also be collectedfrom a surface water source. “Collecting” may include or meanidentifying a previously gathered subterranean water sample, oridentifying a previously determined value of the concentration of theradioactive isotope relative to the stable isotope of the element in thesurface water. A concentration of the radioactive isotope (such as ¹²⁹I)compared to that of the stable isotope (for this case, ¹²⁷I) can bedetermined from prior measurements, e.g., prior to the execution ofprocess 300) of these ratios taken from surface water. The AMS systemdetermines a concentration of a radioactive isotope in the subterraneanwater sample. For example, the AMS system may be operated to determine aconcentration of a particular radioactive isotope, such as ³⁶Cl or ¹²⁹I(or both), in the subterranean water sample (e.g., relative to acorresponding stable isotope of that element). In some aspects, this mayalso include include determining the concentration of the particularradioactive isotope in the surface water sample as well. Alternatively,the concentration of the radioactive isotope in the surface water samplemay be known. The determination of the concentration of the radioactiveisotope with the AMS system may include measuring a ratio of theradioactive isotope (e.g., ³⁶Cl or ¹²⁹I) in the particular water sampleto a stable (non-radioactive) isotope (e.g., ³⁵Cl or ¹²⁷I) of the sameelement (chlorine or iodine, respectively). Thus, reference todetermining a concentration of the radioactive element means, in someaspects, determining a ratio of the radioactive isotope to the stable(non-radioactive) isotope of the same element in the particular (surfaceor subterranean, or both) sample. The AMS system may then compare theconcentrations of the radioactive isotope in the subterranean watersample and the surface water sample. Based on the comparison, the AMSsystem may be used to determine (at least in part) that the subterraneanformation is suitable as a hazardous waste storage repository. Forexample, criteria for determining that the subterranean formation (108or 110 or both) is suitable for the long-term (e.g., 100, 1000, 10,000years or more) storage of hazardous waste material (e.g., spent nuclearfuel) may be the presence of water that has not been exposed to theatmosphere 101 for a particular duration of time, thereby evidencing thesubterranean formation as a geologic formation which does not permitmobile water therethrough, or otherwise allow a flow of liquid from theformation toward the surface water formation 106. Such evidence may beproof of the subterranean formation to store hazardous waste materialwith little to no chance of such material mixing or polluting potablewater fit for human consumption at the surface water formation 106.Based on that determination, the hazardous waste storage repository maybe created (or creation may be initiated) in or under the subterraneanformation (108 or 110 or both).

The present disclosure also describes example implementations of systemsand methods for determining that a subterranean formation (e.g.,formation 108 or 110, both, or another subterranean formation) issuitable as a hazardous (e.g., nuclear or radioactive) waste repositoryformed in deep, human-unoccupiable directional drillholes based on adetermination that fluids (e.g., water, brines, and gases) have beenessentially stagnant in the subterranean formation for periods of tensof thousands to millions of years. For example, when brines in ageologic environment (such as a subterranean formation) are stagnant forlong periods of time and isolated from surface waters, a number ofgeochemical, biological, radiogenic and nucleogenic markers can developwithin the isolated waters that provide independent and corroboratingevidence for the stagnancy and isolation of the brine. Used incombination, these markers can provide a compelling and internallyself-checking system of assessing the suitability of a subterraneanformation for use as a hazardous waste repository formed in deep,directional drillholes.

As an example, there may be indirect indicators of long-term (e.g.,hundreds, to thousands, to tens of thousands, to millions of years)isolation of a fluid (e.g., brine liquid) in a particular subterraneanformation. In some cases, such indirect markers provide evidence forphysical isolation of brines without providing explicit age estimatesfor the duration of brine isolation. Nonetheless, these markers can bepowerful tools for initial screening of potential hazardous wasterepository sites and can be used as part of larger matrix ofself-consistent data providing insight into the appropriateness of arepository site. For instance, strong salinity gradients and highsalinities at depth may indicate density stratification and imply a lackof mixing and/or upward movement of deep waters into shallowerenvironments. Similarly, a low thermal flux through the geologicformation and/or low temperature gradients may suggest a limited or lackof thermally derived buoyant flow upward through a formation. In otherinstances, changes in the stable isotopic composition of the brines, forexample shifts in the deuterium and 18-oxygen isotopic ratios away fromthe Global Mean Water Line, may indicate long term abiogenic water rockinteractions in isolation from surface waters. Similarly, stratificationof different life forms in the shallow vs. deep geo-biosphere withdifferent metabolic requirements and isotopic markers may indicate thatthe different life forms exist in isolation from one another atdifferent depths without mixing and transfer of nutrients from surfacewaters.

One or more of the above indirect markers (and others) may provideconfirmation of: (a) the current lack of physical driving forces (e.g.,saline-density and thermal buoyancy) to cause upward flow and mixing ofthe brines with shallow or surface waters; and/or (b) evidence that inthe past, isolation of the deep brines from surface waters persisted forsignificant amounts of time as reflected in stable isotopic markers forwater rock interaction and existence of distinct depth stratifiedmicrobial populations. These types of indirect markers may providegeneral indications of isolation but may not place strict time limits onthe duration that deep brines have been isolated from the surface. At aminimum, these markers may indicate isolation of some thousands to tensof thousands of years, but may also be consistent with isolation of thedeep brines from the surface environments for millions to >one billionyears. As such, these example indirect markers may provide a secondarybut important and independent system of corroborating geochronologicevidence that places explicit time limits for the isolation of brines atdepth.

As an example, there may be direct geochronologic indicators oflong-term (e.g., hundreds, to thousands, to tens of thousands, tomillions of years) isolation of a fluid (e.g., brine liquid) in aparticular subterranean formation. In order to place explicit timelimits on the past isolation of deep brines, a number of geochronologictools based on the production and decay of different stable and unstableisotopes in the Earth's atmosphere, surface, shallow subsurface (tens ofmeters), and deep subsurface (hundreds to thousands of meters) can bebrought to bear. These geochronologic tools may have independent anddistinct modes of production and together provide an integrated andinternally self-consistent picture of: (a) the downward flow and mixingof surface waters with deep brines by tracing atmospheric and shallowsubsurface derived cosmogenic radioisotopes (e.g., 81Kr, 36Cl, others)with a time frame of, e.g., 1-1.5 million years (Ma); (b) the mediumterm stagnancy and isolation of deep brines with a time frame of, e.g.,100 kiloyears (kyr)-80 Ma, through the measured concentration ofnucleogenic isotopes produced in the deep subsurface (e.g., >100 m) thatdevelop distinct secular equilibrium concentrations over long periods oftime (36Clse 1-1.5 Ma, 129Ise 60-80 Ma, others); (c) the medium to verylong term isolation of brines, e.g., 1 Ma>1 billion years (Ga), from themeasured accumulation of different noble gas isotopes (4He, 40Ar,20-21-22Ne, Kr, 124-139 Xe, and others), each of which has distinctmodes of production in the deep subsurface and provides an independentmeasure of the retentiveness and isolation of deep brines.

As an example, each of the above mentioned geochronologic markers mayprovide an independent assessment of the isolation of deep brines withina subterranean formation and an independent measure of the suitabilityof the formation for use as a hazardous waste repository (e.g., forradioactive or nuclear waste). Used in combination, such geochronologicmarkers can provide a more compelling and complete picture of themobility/stagnancy of brines in the sub surface.

For instance, in an example, a measurement of 81Kr, 36Cl, 20-21-22Neisotopic abundances may be made from a sample of subterranean fluid(e.g., liquid brine) from a formation. The measurement may indicatethat: (1) 81Kr is at or below detection limits of approximately 400atoms/liter, (2) 36Cl concentrations are relatively high but 36Cl/Clratios are consistent with calculated secular equilibrium concentrationvalues for the formation, and/or (3) there is a significant excess of21Ne and a 21Ne/22Ne ratio of approximately 0.5. In such a case, theabsence of 81Kr may be an indicator that there is either insignificantor no downward penetration of surface waters into the deep brine or veryslow exchange and replacement of deep waters by surface waters (e.g.,less than 1% exchange per 10,000 yrs.). The relatively high absoluteconcentration of 36Cl coupled with a low 36Cl/Cl ratio consistent withcalculated 36Cl production in the deep subsurface may indicate that thebrine has been stagnant and isolated from the surface for a minimum of1-1.5 Ma. The accumulation and retention of 21Ne in the formation and21Ne/22Ne ratio may indicate that the neon has been produced andretained in the deep formation for >100 Ma. Each of these isotopicsystems represents a different and unique marker, e.g., of mode ofproduction, and of estimate of isolation. That all three indicatorsagree may also provide cross checking of the markers and contributes tothe overall certainty of the result.

In some aspects of the present disclosure, indirect markers of isolationmay be used as proxies for direct markers. For example, in addition tothe direct geochronologic measures, indirect markers of the isolationage of the brines such as (1) salinity-density stratification withdepth, (2) distinct changes in stable isotopic measures such as oxygenand deuterium with depth, as well as (3) changes in the type, metabolicprocesses, and diversity of life forms as a function of depth, amongother indicators, may serve as secondary supporting evidence and proxiesfor the long term isolation of deep brines. In some aspects, measuringthese physical and chemical properties may be simpler, faster, and lessexpensive than performing detailed geochronologic age determinations. Insome example aspects, simply collecting data on salinity, temperatureand other proxies such as oxygen and deuterium isotopes can provide agood initial screening for a hazardous material repository site.

For example, if a potential hazardous waste repository site hasrelatively high heat flow and low salinity gradients as a function ofdepth, this may be an indication that the site has significant upwellingpotential and relatively low probability of being appropriate for anuclear waste repository. In contrast, if a site has relatively lowthermal gradient and relatively steep increases in salinity atincreasing depths this may correlate with stagnancy and isolation ofbrines at depth. If oxygen and deuterium (or other) isotopes furthercorrelate with salinity, this may provide another indication that thebrines at depth are density stratified and have been isolated over longtime periods. This type of screening allows a number of sites (e.g., fora hazardous waste repository) to be initially evaluated in a relativelyrapid manner and limit costs associated with difficult drilling,sampling and measuring processes associated with data on a broader suiteof isotopes (e.g., 81-Kr, noble gases, and other isotopes). This initialscreening may leverage the use of indirect proxies of isolation as atool for locating and concentrating efforts on the most feasible sites,while saving costs and time associated with full detailed geochemicaland isotopic measurements.

In some aspects, such implementations can be applied in subterraneanformations that have natural uranium or thorium (or both) in thesubterranean formation. Many sedimentary, metamorphic, and igneousrocks, including shale formations and crystalline basement rock, havesuitable levels of uranium or thorium (or both) for the describedimplementations.

Turning now to FIG. 3A, this figure illustrates an example process 300for determining the suitability of a subterranean formation as ahazardous waste repository by an analysis of a production andaccumulation of one or more noble gas isotopes (and possibly othernuclides) in a subterranean formation based on a natural decay of one ormore of uranium, thorium, or potassium (and possibly other secondarynucleogenic reactions). In some aspects, process 300 may be implementedby or with the system 100, including the fluid testing system 122.

Process 300 may be based on natural radioactive decay that occurs indeep subterranean formations and leads to the production of a variety ofisotopes that were not present, or present in a lesser degree, in theoriginal rock of the subterranean formation. For example, the alphadecay of uranium and thorium produces an alpha particle that consists oftwo neutrons and two protons. Additional alpha particles are producedfrom the radioactive decay of the uranium and thorium decay products.When one atom of U-238 decays, there are seven additional alphaparticles produced from the decay products. Most of these alphaparticles come to rest in the rock of the subterranean formation.Because of their strong positive charge, these alpha particles take theouter electrons from other atoms. The alpha+electrons are helium atoms,and these can accumulate in the rock. In many porous rocks, thisproduced helium diffuses away, and is often captured under “cap”formations that consist of impermeable rock. Such accumulations can beharvested, and they provide essentially all of commercial helium.

If, however, the rock has very low permeability, then the helium can betrapped in the rock. A measure of helium concentration, compared withthe expected helium production from the uranium and thorium, may providea measure of the retentiveness of the rock against gas diffusion ortransportation by moving fluids. If, for example, the rock containsenough helium that it would have taken 10 million years to produce itfrom the uranium and thorium, then the ability of the subterraneanformation to retain helium gas is approximately 10 million years. Such adetermination can provide for a determination that the subterraneanformation is suitable as a hazardous waste repository (e.g., formed in adeep, directional drillhole), since diffusion of one or more radioactiveisotopes in nuclear or radioactive waste is known to be slower than thatof helium.

Process 300 may begin at step 301, which includes forming a testdrillhole from a terranean surface to a subterranean formation. Forexample, test drillhole 104 may be formed (e.g., drilled) from theterranean surface 102 to the subterranean formation 110. Process 300 maycontinue at step 302, which includes collecting a subterranean watersample from the subterranean formation. In some aspects, the watersample is brine. Alternatively, step 302 could include collecting asubterranean fluid sample. In some aspects, steps 301 and 302 may bereplaced or avoided by identifying a previously collected subterraneanfluid sample (e.g., liquid brine).

Process 300 may continue at step 303, which includes determining aconcentration of at least one noble gas isotope in the subterraneanwater sample. For example, in the example of helium, a heliumconcentration may be determined from a liquid (e.g., brine) sample takenfrom the subterranean formation. For instance, helium is a dissolved gasin the brine, and standard downhole tools may be used to obtain a brinesample from the subterranean formation. If, however, the subterraneanformation has very low brine flow rates (e.g., due to the pressure ofthe subterranean formation), a rock core sample that includes brine maybe collected. Once the brine sample is collected (either directly orfrom a core sample), gas extraction methods may be used to collect thehelium within the brine sample. In some aspects, the concentration ofhelium may depend on the rock type within the subterranean formation.

Process 300 may continue at step 304, which includes determining aproduced amount of the noble gas isotope in the subterranean formationbased on a production rate of the noble gas isotope and a minimumresidence time. For instance, continuing the example of helium, once theconcentration is determined (e.g., by known sampling techniques) fromthe brine sample, a production amount of the helium in the subterraneanformation during an assumed or desired residence time (in atoms) isdetermined. Although a production rate (atoms per year) of helium in asubterranean formation may vary with geology, this rate is a function ofthe concentrations of uranium and thorium in the formation (i.e., anamount of uranium and thorium per unit volume of formation). In someaspects, the concentration of uranium and/or thorium, and thus theproduction rate of helium (or another noble gas isotope), may be knownor determined by the known bulk rock chemistry of the subterraneanformation. In some aspects, the residence time represents a time periodin which the helium was fluidly isolated in the subterranean formation,as well as a time period in which it is desired for hazardous waste tobe stored and fluidly isolated within the subterranean formation, e.g.,thousands of years, tens of thousands of years, millions or years, orother time period.

Process 300 may continue at step 305, which includes calculating a ratioof the determined concentration to the determined produced amount of theat least one noble gas isotope. For example, continuing with the heliumexample, the determined concentration represents an actual amount (e.g.,concentration) of helium in the subterranean formation being examined asa potential hazardous material repository. The determined producedamount represents, e.g., the theoretical amount (e.g., concentration) ofhelium that should be present in the subterranean formation at aparticular minimum residence time of the fluid sample from theformation. The particular minimum residence time, as described, may beselected, e.g., based on a desired amount of time for which hazardouswaste stored in the subterranean formation is isolated (e.g., from asource of mobile water) within the subterranean formation. This timeperiod could be, for example, tens of years, hundreds of years,thousands of years, or more (or, in some cases, less).

Process 300 may continue at step 306, which includes determining thatthe subterranean formation is suitable as a hazardous waste storagerepository based, at least in part, on the calculated ratio being at ornear a threshold value. In some aspects, the threshold value may be 1 orvery close to 1, thereby showing that the helium concentration in thesubterranean fluid sample has been isolated within the formation for ator near the desired minimum residence time duration. For example, if thecalculated ratio is much less than 1, the determined concentration ismuch less than the determined produced amount, thereby showing thathelium has escaped the subterranean formation (i.e., has not beenfluidly isolated from other formations, including possibly, formationswith mobile or surface water). If the calculated ratio is greater than1, the determined concentration is greater than the determined producedamount, thereby showing that helium has not escaped the subterraneanformation (i.e., has been fluidly isolated from other formations,including possibly, formations with mobile or surface water).

Process 300 may continue at step 307, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the subterranean formationbased on the determination of step 306. Process 300 may continue at step308, which includes storing hazardous waste material in the hazardouswaste storage repository. Example methods and processes for steps 307and 308 are described with reference to FIGS. 2A-2B and 4.

In some aspects, process 300 includes repeating steps 303-305 but for adifferent noble gas isotope than a noble gas isotope original tested inthe subterranean fluid sample. For example, as described, helium is aparticular noble gas isotope with a particular production mode. Thefluid sample may also be tested for a different noble gas isotope thanhelium, which may have a different production mode than helium. In someaspects, another ratio of a determined concentration to a determinedproduced amount of the other noble gas isotope is calculated andcompared against the threshold value. If the other calculated ratio isat or near the threshold value, this determination may be furtherconfirmation that the subterranean formation is suitable as a hazardouswaste storage repository. The additional calculated ratio of thedetermined concentration to the determined produced amount of the othernoble gas isotope can also be compared to the first calculated ratio.This comparison (e.g., if the calculated ratios are equal or close toequal) may also be further confirmation that the subterranean formationis suitable as a hazardous waste storage repository.

As described, different noble gas isotopes may have different productionmodes; an example production mode for helium was previously described.In alternative aspects, when alpha particles from uranium or thoriumdecay are emitted, instead of stopping and becoming the core of heliumatoms, they frequently collide with the nuclei of other elements (e.g.,aluminum, silicon, magnesium, oxygen and fluorine), and in rareinstances (less than 1:10 million) result in the capture of alphaparticle by the element. The capture of alpha particles can result inthe production of other rare noble gas isotopes which can also be usedto determine the length of time these rare isotopes have beenaccumulating in the subsurface.

As an illustrative example, consider the noble gas isotope of neon,which has three stable isotopes: neon-20 (Ne-20), neon-21 (Ne-21), andneon-22 (Ne-22). The natural concentrations of these isotopes are90.48%, 0.27%, and 9.25%, respectively. However, when neon is exposed tosubsurface alpha radiation over millions of years, the absorption of thealpha particles (primarily by oxygen, fluorine and magnesium isotopes)drastically changes both the absolute concentrations of the isotopes andtheir ratios relative to one another. The rate of production of thedifferent neon isotopes depends on the concentration of the targetisotopes (17O, 18O, 19F in this instance) within a ˜40 micron radius ofthe U or Th emitter. The principal nuclear reactions are 17O(α,n)Ne-20,18O(α,n)Ne-21, and 19F(α,n)Ne-22. Production of neon isotopes through(α,n) reactions in the subsurface dramatically favor the production ofNe-21 relative to its original isotopic abundance. As a result, overlong time periods the absolute and relative abundances of Ne-21 change.In deep subterranean formations, the Ne-21/Ne-22 ratio has been observedto grow from 0.027 to 0.6, a 20 fold increase. Such dramatic changes inisotopic composition provide evidence for tens to hundreds of millionsof years of an accumulation of Ne-21. Since the excess was producedafter the rock in the subterranean formation was formed, the presence ofthis excess of Ne-21 is a measure of the retention capability of thesubterranean formation for the gas, neon.

Additional changes in the concentration of other noble gases, e.g.,argon, krypton and xenon, produced by a number of subsurface nucleogenicreactions (beta decay of 40K to 40Ar, fission of 238U for the productionof Xe isotopes, and other reactions) also contribute information on theisolation and stagnancy of the brines. Thus, the measurement of ratiosof the isotopes of the noble gases helium, argon, neon, krypton, andxenon offer an estimation of the fluid isolative capabilities of thesubterranean formation. Thus, in some aspects, such an estimation mayprovide a determination that the subterranean formation is suitable as ahazardous waste repository for, e.g., radioactive waste. Because each ofthese noble gas isotopic systems has a different mode of production,each provides an independent estimate of the fluid isolative capacity ofdeep brines. The use of multiple noble gas isotopes as independentgeochronologic markers to estimate the age and isolation of brine indeep geologic formations provides a powerful and integrated system ofcross checking and validating the age and isolation estimates.

The present disclosure also describes a process for determining thesuitability of a subterranean formation as a hazardous waste repositorybased on a measurement of a krypton isotope from a fluid sample from thesubterranean formation. For example, turning to FIG. 3B, this figureillustrates an example process 310 for determining the suitability of asubterranean formation as a hazardous waste repository based on ameasurement of a krypton isotope from a fluid sample from thesubterranean formation. In some aspects, process 310 may be implementedby or with the system 100, including the fluid testing system 122.

For example, in example implementations, a measurement of an amount ofkrypton-81 (Kr-81) in a subterranean formation (e.g., within a liquidsuch as brine in the formation) may be used to determine the suitabilityof the subterranean formation as a hazardous waste repository. Forexample, in deep subterranean formations, another measure of a fluidisolative capability of the formation may include a determination thatsurface water (e.g., from a body of water on a terranean surface or ashallow aquifer) are not appreciably mixing with brine in thesubterranean formation. One measure of this isolation is the absence ofradionuclides produced in the earth's atmosphere or near subsurface atdepth. One example radioisotope to demonstrate this isolation is thespecific noble gas krypton-81 (half-life 229,000 years), which isproduced at the Earth's surface, is incorporated and travels withsurface waters, and has no significant subsurface production mode. Ifsurface waters are mixing significantly with brine in the subterraneanformation, Kr-81 would be present in the brine in measurable quantities.If no Kr-81 is observed within the brine in the subterranean formation,then the age limit will be determined by the precision with which thatzero level is determined. This age limit may be about 1.5 million years.

Thus, the absence of measurable Kr-81 in a subterranean formation atcurrent measurement sensitivities indicates that the brine in thatsubterranean formation has been fluidly isolated from the terraneansurface or shallow aquifers for 1.5 million years or more. Thus,measurement of Kr-81 in a subterranean formation may offer an estimationof the fluid isolative capabilities of the subterranean formation. Thus,in some aspects, such an estimation may provide a determination that thesubterranean formation is suitable as a hazardous waste repository for,e.g., radioactive waste.

Process 310 may begin at step 311, which includes forming a testdrillhole from a terranean surface to a subterranean formation. Forexample, test drillhole 104 may be formed (e.g., drilled) from theterranean surface 102 to the subterranean formation 110. Process 310 maycontinue at step 312, which includes collecting a subterranean watersample from the subterranean formation. In some aspects, the watersample is brine. Alternatively, step 312 could include collecting asubterranean fluid sample. In some aspects, steps 311 and 312 may bereplaced or avoided by identifying a previously collected subterraneanfluid sample (e.g., liquid brine).

Process 310 may continue at step 313, which includes determining aconcentration of a krypton isotope in the subterranean water sample. Forexample, in some aspects, a concentration of Kr-81 may be determinedfrom the water (or fluid) sample. In some aspects, as Kr-81 may be veryscarce, the water sample may be on the order of 10-100 liters. The Kr-81concentration may be measured using Atom Trap Trace Analysis (ATTA), asystem that traps individual atoms of Kr-81 in the fluid sample andcounts these atoms.

Process 310 may continue at step 314, which includes determining thatthe concentration of the krypton isotope is less than a threshold value.For example, once the concentration of the Kr-81 in the subterraneanfluid sample is determined, it may be compared to a known concentrationof Kr-81 in surface water samples (e.g., a threshold value). Forexample, in a surface water sample, an average Kr-81 concentration maybe about 1300 atoms/liter. After about 4-5 half-lives (and accountingfor an 80% extraction efficiency), there may be about 60 atoms to remainper liter.

Process 310 may continue at step 315, which includes determining thatthe subterranean formation is suitable as a hazardous waste storagerepository based, at least in part, on the concentration being less thatthe threshold value. For example, in order to determine if thesubterranean fluid sample concentration is low enough to show that thesubterranean formation has suitable fluid isolative capabilities (i.e.,that the Kr-81 has been isolated in the subterranean formation away froma mobile water or surface water formation). ATTA may use 100 litersamples, so in the example case in which the subterranean formation issuitable for a hazardous waste repository, there would be about 6000atoms of Kr-81 remaining. Counting efficiency in ATTA is about 1-2%;thus, of the 6000 atoms remaining, the ATTA would count about 60-120atoms from the brine sample. Such a concentration of Kr-81 in the brinesample from the subterranean formation (i.e., 60-120 count) effectivelyshows that the subterranean formation is suitable as a hazardous wasterepository in accordance with the Kr-81 analysis described herein.

Process 310 may continue at step 316, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the subterranean formationbased on the determination of step 315. Process 310 may continue at step317, which includes storing hazardous waste material in the hazardouswaste storage repository. Example methods and processes for steps 316and 317 are described with reference to FIGS. 2A-2B and 4.

The present disclosure also describes analyses of additional isotopesthat are formed through subsurface nuclear processes such as Cl-36,I-129, and Ar-39 and can provide still other independent andcomplementary means of assessing the age and isolation of brines. Forexample, when alpha particles from uranium or thorium decay are emitted,instead of stopping and becoming the core of helium atoms, theyfrequently collide with the nuclei of other elements (e.g., aluminum,silicon, magnesium, oxygen and fluorine), and cause the release ofneutrons from these target elements. Measurements of the uranium andthorium concentrations, combined with estimates or measurements of bulkrock chemistry, allow for the estimation of a neutron flux within thesubterranean formation. A produced neutron could come to rest and decay(with a half-life of 10.3 minutes), but more typically, the producedneutron is absorbed on another nucleus in the formation. The result ofthe absorption is a new isotope.

When neutrons are absorbed on some stable nuclei, they can createunstable, that is, radioactive elements. For example, a neutron absorbedon the nucleus of chlorine-35 (Cl-35) (the most abundant stable isotopeof chlorine) forms chlorine-36 (Cl-36), a radioactive atom with ahalf-life of 301,000 years. If none of the chlorine is carried away, ordiffuses away through the rock in the formation, then the concentrationof Cl-36 will continue to accumulate until the production rate (fixed bythe neutron flux and Cl-35 concentrations) matches the decay rate (whichincreases proportional to the amount of Cl-36 presents). This isotopicequilibrium ratio is reached asymptotically, but most of it isaccomplished after 4 to 5 half-lives of Cl-36, that is, after 1.2-1.5million years.

This isotopic equilibrium ratio of Cl-35 and Cl-36 can be calculatedfrom the abundances of Cl-35, uranium and thorium, and from the chemicalcomposition of the rock in the subterranean formation. However, thisisotopic equilibrium ratio will not be reached if the chlorine isescaping from the subterranean formation (e.g., in liquid that is mobileand, for instance, moves toward the terranean surface). In that case, alower value for the ratio will indicate that the residence time of thechlorine atom in the formation is less than 1.5 million years. In someaspects, such a ratio may be informative for the determination ofwhether or not the subterranean formation is suitable as a hazardouswaste repository, since Cl-36, itself, is a threat to human health fromleakage of underground waste. Thus, measurement of ratios of theisotopes of chlorine offer an estimation of the fluid isolativecapabilities of the subterranean formation. Thus, in some aspects, suchan estimation may provide a determination that the subterraneanformation is suitable as a hazardous waste repository for, e.g.,radioactive waste.

Other isotopes can also be used in a similar fashion. One of these isiodine-129 (I-129), which has a half-life of 16 million years. Thisisotope is produced underground by fissions of uranium and thorium.I-129 reaches a steady state level in 4-5 half lives, or 60-70 millionyears. Deviation from this steady-state level may show that the iodineis diffusing or being carried away on a time-scale of several millionyears or less. Thus, measurement of ratios of the isotopes of iodineoffer an estimation of the fluid isolative capabilities of thesubterranean formation. Thus, in some aspects, such an estimation mayprovide a determination that the subterranean formation is suitable as ahazardous waste repository for, e.g., radioactive waste.

According to example embodiments, determination of isotopic equilibriumratios as described herein (e.g., with respect to noble gases, chlorine,iodine as examples) may include accurately calculating neutron flux(e.g., by the fluid testing system 122). For example, neutron flux canbe determined by determining a gamma ray flux (for example, by downholegamma ray logging instruments). The determined gamma ray flux can thenbe used to estimate a uranium or thorium decay rate (or both). From thecalculated decay rate (or rates), an alpha-particle flux can bedetermined. From one or more rock samples collected from thesubterranean formation, measurements of an abundance of the alphatargets that create the most neutrons (e.g., one or more of aluminum,silicon, magnesium, or oxygen) are taken. From these measurements, theneutron flux can be calculated with sufficient precision to allow theisotopic equilibrium ratio determination to be used to then estimate thefluid isolative capability of the subterranean formation.

In some aspect, a high precision of the neutron flux determination isuseful. Direct measurement of neutron fluxes in the subterraneanformation using neutron monitors could reduce uncertainties. In someaspects, the neutron flux may be too low for such a system to bepractical and typically requires months of observation.

In some aspects of the present disclosure, accurate neutron flux may bedetermined by determining a concentration of a nucleonic isotopeproduced in the subterranean formation that is also radioactive and hasa short enough half-life that the isotope will reliably be in a state ofisotopic equilibrium when measured. For instance, since such ameasurement determines neutron flux due to the short time in whichequilibrium is reached, there may be very little loss to flow.

In an example implementation, the isotope is argon-39 (Ar-39). Argon-39is produced from neutron capture reaction, ³⁹K(n,p)³⁹Ar, and has ahalf-life of 269 years. Potassium-39 (K-39) is the most abundantpotassium isotope (93%) and is among the most common constituents ofrock forming minerals. Potassium-39 concentrations, coupled withmeasurements of Ar-39 and with a known neutron capture cross section ofK-39, provide a direct measure of the subsurface neutron flux, averagedover approximately 1500 years, the time needed to reach the isotopicequilibrium concentration of argon-40 (Ar-40). This neutron fluxestimate established by direct measurement can then be applied tocalculations for production rate of longer-lived radionuclides (e.g.,Cl-36) and of other nucleogenic stable isotopes. This method offers aconsiderable improvement over both first principle estimates of neutronfluxes and inferred neutron flux estimates based on gamma rays.

In some aspects, the neutron flux is an intermediate parameter in thedetermination according to the described implementations. Thus, ineffect, it is the measurement of the ratio of Ar-39 to other isotopesthat determines the presence of isotopic equilibrium and thus ofsuitability of the formation for use as a hazardous waste repository.

The present disclosure also describes a process for determining thesuitability of a subterranean formation as a hazardous waste repositorybased on a measured neutron flux of one or more isotopes. For example,turning to FIG. 3C, this figure illustrates an example process 320 fordetermining the suitability of a subterranean formation as a hazardouswaste repository based on a calculated neutron flux of a particularisotope and a predicted production rate of a second isotope that isbased on the calculated neutron flux. The predicted production rate iscompared against a concentration of the second isotope found in asubterranean fluid sample. In some aspects, process 320 may beimplemented by or with the system 100, including the fluid testingsystem 122.

Process 320 may begin at step 321, which includes forming a testdrillhole from a terranean surface to a subterranean formation. Forexample, test drillhole 104 may be formed (e.g., drilled) from theterranean surface 102 to the subterranean formation 110. Process 320 maycontinue at step 322, which includes collecting a subterranean watersample from the subterranean formation. In some aspects, the watersample is brine. Alternatively, step 322 could include collecting asubterranean fluid sample. In some aspects, steps 321 and 322 may bereplaced or avoided by identifying a previously collected subterraneanfluid sample (e.g., liquid brine).

Process 320 may continue at step 323, which includes determining aneutron flux of a first isotope in the subterranean formation. In someaspects, the first isotope may be one of Ar-39, Fe-59, Co-60, Ni-63,Kr-85, Ni-63, or C-14. In some aspects, the neutron flux may bedetermined according to the bulk rock chemistry of the subterraneanformation (e.g., an amount of uranium or thorium (or both) per unitvolume of the subterranean formation). In some aspects, a neutron fluxvalue may be determined for a (relatively) short-lived radioisotope,such as Ar-39, which is produced from K-39 through neutron capture fromthe release of alpha particles from uranium or thorium decay (or both).Thus, the bulk rock chemistry of the subterranean formation, whichindicates an amount of uranium or thorium in the subterranean formation,indicates an amount of neutrons that are released to produce Ar-39 fromK-39. From this information, the neutron flux value for Ar-39 can bedetermined.

Process 320 may continue at step 324, which includes calculating, basedat least in part on the determined neutron flux, a predicted productionrate of a second isotope in the subterranean formation. In some aspects,the second isotope is Cl-36. A stable form of Cl-36 is Cl-35. In anyevent, in some aspects, the first isotope has a shorter half-life (e.g.,in years) than the second isotope. For instance, once the neutron fluxfor, e.g., Ar-39, is determined, this value can be used to determine apredicted production rate of other, longer lived, substances in thesubterranean formation, such as, for example, radioisotopes like Cl-36.

Process 320 may continue at step 325, which includes calculating a firstratio of the predicted production rate of the second isotope relative toa theoretical production rate of a stable form of the second isotope. Insome aspects, a ratio of the predicted production rate of, e.g., Cl-36,with the predicted production of the respective stable (or other)isotope, e.g., Cl-35 (or Cl). The predicted production rate, in thisexample, of the stable isotope is based on the bulk rock chemistry ofthe subterranean formation and assumes no migration of the stableisotope out of the subterranean formation (i.e., assumes thesubterranean formation is fluidly isolative of the stable isotope).

Process 320 may continue at step 326, which includes measuringrespective concentrations of the second isotope and the stable form ofthe second isotope in the subterranean water sample. For example, thesubterranean water (or fluid) sample from the subterranean formation ofstep 344 is measured to determine concentrations of the longer livedisotope and a stable form of the longer lived isotope. For example,concentrations of, e.g., Cl-36, as well as, e.g., Cl-35 (or Cl),respectively, are determined.

Process 320 may continue at step 327, which includes calculating asecond ratio of the measured concentration of the second isotoperelative to the measured concentration of the stable form of the secondisotope. For example, a ratio of the measured concentration of, e.g.,Cl-36 with the measured concentration of the respective stable (orother) isotope, e.g., Cl-35 (or Cl) is determined.

Process 320 may continue at step 328, which includes determining thatthe subterranean formation is suitable as a hazardous waste repositorybased at least in part on a comparison of the first and second ratios.For instance, as the first ratio represents a ratio that assumes thatthe subterranean formation isolates fluid therewithin from otherformations for a sufficient period of time (e.g., tens, hundreds,thousands, or millions of years), if the comparison of step 356 is equalor within a sufficiently small deviation from equal, the subterraneanformation may be determined to be suitable for a hazardous wasterepository.

Process 320 may continue at step 329, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the subterranean formationbased on the determination of step 328. Process 320 may continue at step330, which includes storing hazardous waste material in the hazardouswaste storage repository. Example methods and processes for steps 329and 330 are described with reference to FIGS. 2A-2B and 4.

Thus, implementations of the present disclosure according to FIG. 3C usea short-lived radioisotope, which is created by local neutron fluxes butwhich exists for such a relatively short time that it is not depleted byflow or by diffusion, for the purpose of providing a calibration for theinterpretation of longer-lived species, and for the determination ofwhether these long-lived species are in isotopic equilibrium. Otherrelatively short-lived isotopes could be used in place of Ar-39, such asFe-59 (44.5 day half-life), Co-60 (5.3 year half-life), Ni-63 (100 yearhalf-life), Kr-85 (half-life of 10.8 year half-life), nickel-63 (100year half-life), carbon-14 (5730 year half-life), and others that areproduced when neutron collide with elements in the rock of thesubterranean formation.

Turning now to FIG. 3D, this figure illustrates an example process 340for determining the suitability of a subterranean formation as ahazardous waste repository by an analysis of concentrations of one ormore radioisotopes entrained in subterranean fluid (e.g., brine) withinsubterranean formations located at different vertical depths below aterranean surface to show mobility (or lack thereof) of the subterraneanfluid. In some aspects, process 340 may be implemented by or with thesystem 100, including the fluid testing system 122.

For example, process 340 may include measurements of concentrations ofcertain radioisotopes at depth that can be used as an indicator thatwater has not been transported to that depth from the terranean surfacefor thousands to millions of years. For instance, several radioisotopesare produced in the biosphere by cosmic rays. Such radioisotopes lowerconcentration or absence at depth indicates that their transport fromthe surface has taken many half-lives. Example radioisotopes for thisapproach include C-14, Cl-36, Kr-81, and I-129.

These examples, however, may give limits on the downward migration ofradioisotopes, not on the upward migration. The two limits may belinked, but it is also advantageous to make a determination of watermobility in a subterranean zone that places limits on the upwardmigration of, e.g., radioisotopes.

Generally, process 340 describes an example implementation that includesmeasuring a concentration of one or more radionuclides per volume (e.g.,liter) of water as a function of depth from a terranean surface and,based on the measurement(s) determining an upward water mobility in asubterranean zone. For example, radioisotopes deep in the earth comefrom several sources. Some examples include: (1) Radioisotopes producedin the biosphere that have migrated to depth; (2) Primordialradioisotopes, created in the formation of the solar system, and whichare still present in geologic formations because of their longhalf-lives, and include, e.g., U-238, U-235, Th-232, and K-40; (3)Secondary radioisotopes produced deep in the Earth from the primordialradioisotopes, including I-131 (a fission fragment from uranium andthorium fission) and radioisotopes produced from neutron absorption,including Cl-36.

In some aspects of process 340, radioisotopes of either (1) or (2), orboth, may be measured. Measurements on such radioisotopes can providedeterminations and limits on upward radioisotope migration. Thisdetermination can then be used to determine the suitability of asubterranean zone from which the water sample was taken as a hazardouswaste repository in which hazardous waste (e.g., nuclear waste such asspent nuclear fuel or high level waste) can be stored in deep,human-unoccupiable drillholes.

Process 340 may begin at step 341, which includes forming a testdrillhole from a terranean surface through first and second rock layers(i.e., below a terranean surface). For example, test drillhole 104 maybe formed (e.g., drilled) from the terranean surface 102 to and throughsubterranean formations 108 and 110. In some aspects, the first andsecond rock layers are located in adjacent (e.g., contacting)subterranean formations. In some aspects, the first and second rocklayers may be separated by one or more intervening subterraneanformations. In some aspects, the first and second rock layers maycomprise a single rock type, but in distinct layers.

Process 340 may continue at step 342, which includes collecting a firstwater sample from the first rock layer at a first depth below theterranean surface. In some aspects, the water sample is brine.Alternatively, step 342 could include collecting a fluid sample. In someaspects, steps 341 and 342 may be replaced or avoided by identifying apreviously collected water sample (e.g., liquid brine) from the firstrock layer.

Process 340 may continue at step 343, which includes determining a firstconcentration of a particular radioisotope in the first water sample. Insome aspects, the particular isotope may be uranium (U) and/or thorium(Th). In some aspects, the concentration can be determined using gammaray logs, e.g., using downhole wellbore logging tools that measurenatural gamma rays as a function of depth from the terranean surface.Such logs are typically interpreted as evidence of rock type, since somerocks (e.g., shale) typically have higher concentrations of theseisotopes than do other rock types (e.g., limestone). Some of the gammascome from K-40 (high in clay) and others come from uranium and thorium.In some aspects, for the purpose of isolation determination, gamma raylogs may be used as an initial indicator of a degree of upward watermobility in a rock formation. Thus, step 343 may also includedetermining a concentration of U and/or Th from a core sample taken fromthe first rock layer.

As another example, the particular radioisotope may be potassium 40,abbreviated as K-40, which also serves as a tracer for elements thathave similar transport properties to potassium. Step 343 may thereforeinclude determining a concentration of K-40 in the first water sample.

As other examples, both Cl-36 and I-129 may be the particularradioisotope. These radioisotopes are produced underground due to thespontaneous fission of U-238. Thus, high levels of Cl-36 and I-129 areexpected to be found in formations, such as shale, that are high inuranium. The iodine is typically a direct fission fragment, while thechlorine is produced when a neutron from the fission is absorbed ontothe abundant and stable isotope, Cl-35. Step 343 may therefore includedetermining a concentration of Cl-36 and/or I-129 in the first watersample.

Process 340 may continue at step 344, which includes collecting a secondwater sample from the second rock layer at a second depth below theterranean surface that is deeper than the first depth. In some aspects,the water sample is brine. Alternatively, step 344 could includecollecting a fluid sample. In some aspects, step 344 may be replaced oravoided by identifying a previously collected water sample (e.g., liquidbrine) from the second rock layer.

Process 340 may continue at step 345, which includes determining asecond concentration of the particular radioisotope in the second watersample. Step 345, in some aspects, may be similar to step 343, but withthe second water sample rather than the first. Thus, subsequent to step345, there are two concentrations determined of the particularradioisotope at two different depths under the terranean surface.

Process 340 may continue at step 346, which includes determining thatthe second rock layer is suitable as a hazardous waste storagerepository based at least in part on the second concentration beinggreater than the first concentration by a specified percentage. In someaspects, the specified percentage may be, e.g., 50% or greater.

Process 340 may continue at step 347, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the second rock layer basedon the determination of step 346. Process 340 may continue at step 348,which includes storing hazardous waste material in the hazardous wastestorage repository. Example methods and processes for steps 347 and 348are described with reference to FIGS. 2A-2B and 4.

As described, example implementations of process 340 indicate—by ashowing that a particular radioisotope is more highly concentrated in adeeper rock layer relative to a shallower rock layer—that the deeperrock layer has fluidly isolated the particular radioisotope in thedeeper rock layer for a desired time duration (e.g., tens, hundreds,thousands, or millions of years). Such a showing may indicate that thedeeper rock layer is suitable as a hazardous waste repository.

For example, in the case of the radioisotope being K-40, the presence ofa first rock layer having a high concentration of K-40 (measured inatoms per volume of water) with a second rock layer with a lowconcentration of K-40 above the first rock layer provides an indicationthat the K-40 in the first rock layer is not upwardly mobile. As anotherexample, a presence of a deep layer with high uranium/thorium that isbelow a layer that is low in these elements may indicate little to noupward water (i.e., containing the U or Th) mobility in the deeperlayer. For example, the presence of a low U/Th concentration layer abovea layer with a high U/Th concentration gives an indication that uraniumand thorium have not been migrating upward for periods of millions ofyears. Such a determination is useful for showing that uranium andthorium isotopes, if released from disposed fuel, would not migratequickly to the surface. Besides uranium or thorium, other radioisotopes,which have similar chemistry to that of uranium and thorium, such asAmericium, may also be measured as described above.

As another example, an estimation of upward water flow can be made bymeasuring the Cl-36 and I-129 levels in rock formations that lie above aformation in which hazardous waste is to be stored. Since the mainmechanism for transport of these isotopes may be transport by movingwater, the measurement should be made as concentrations (number ofatoms) per volume of water in the selected formation(s). A concentrationin a first rock layer that is substantially less than the concentrationin a second, deeper rock level is an indicator that water transport fromthe second rock layer to the first layer takes more time than thehalf-lives of the measured radioisotopes. In some aspects, a flow ofboth Cl-36 and I-129 might be slower than a flow of the water thatcarries them, because chlorine and iodine can be absorbed for varyingperiods of time on the surrounding rock and later released. However, forthe disposal of nuclear waste, the important number is not the velocityof flow of the water (which may be harmless) but the velocity of flow ofthe radioisotopes. In particular, the radioisotopes Cl-36 and I-129 areof concern since they are abundant in nuclear waste and long lived. Forthat reason, the measurement of upward flow of naturally occurring Cl-36and I-129 is even more directly relevant to the suitability of asubterranean formation to safely (e.g., without upward leakage) storesuch nuclear waste (e.g., for hundreds if not thousands or millions ofyears) than is the actual flow of water (or other fluid).

Turning now to FIG. 3E, this figure illustrates an example process 350for determining the suitability of a subterranean formation as ahazardous waste repository by determining a secular equilibrium of oneor more radioisotopes entrained in subterranean fluid (e.g., brine)within subterranean formations located at different vertical depthsbelow a terranean surface to show mobility (or lack thereof) of thesubterranean fluid. In some aspects, process 350 may be implemented byor with the system 100, including the fluid testing system 122.

For example, process 350 may analyze concentrations of natural isotopesof chlorine, iodine, and helium to put limits on the movement ofsubterranean fluid for the purpose of determining whether a particularsubterranean formation (i.e., rock layer) is suitable as a hazardouswaste repository for nuclear waste. Measurements of vertical profiles ofthese isotopes can determine the rate of the vertical flow of brines inthe past. Such a measure may help determine whether the formation issuitable as a hazardous waste repository, e.g., if the rate is lowenough to show that any mobile water/brine will not carry leaked nuclearwaste toward a terranean surface.

As described, rock formations continue trace levels of U and Th. Theprimary components of these elements are radioactive with half-lives of4.5 billion years (U-238) and 14 billion years (Th-232). Theseradioisotopes decay primarily by alpha emission, although there is acontribution from spontaneous fission of uranium isotopes 238 and 235.The radioactive decay of these isotopes has been constant over millionsof years because of the long half-lives. The radioactive decays resultin the constant rate of production of secondary radioisotopes.

In particular, U and Th lead to the constant production of chlorine-36(Cl-36) and iodine-129 (I-129). The production of Cl-36 is complexprocess. The main production is a result of several steps: a uranium orthorium atom undergoes radioactive decay and emits an alpha particle.The alpha collides with another element (such as sodium or magnesium)present in the rock, and that often results in neutron emission.Finally, the neutron is absorbed by the stable isotope of chlorine,Cl-35, to produce the radioactive isotope Cl-36.

If Cl-36 were stable, its contribution would continue to build. However,it is radioactive with a half-life of 300,000 years. This decay is aprimary source of depletion for Cl-36, and if none escapes a particularrock layer, the level of Cl-36 in that rock layer reaches a valuereferred to as “secular equilibrium” in which the production rate equalsthe decay rate. Secular equilibrium has been observed in water that hasbeen trapped and isolated in deep subterranean formations. The observedlevel of Cl-36 in these brines is consistent, within about a 30%accuracy with the level expected by calculating the secular equilibriumlevel using the chemical and isotopic constitution of the rock and thewater. The equilibrium value would shift, however, if there were a lossof Cl-36, for example, by fluid flow, i.e., “flow leakage.”

Because of the hydrostatic equilibrium, any leaked brine is replaced bynew brine. The shift could reduce the level of Cl-36 if the brinereplacing the leaked component come from a rock with a lower productionrate, either because uranium and thorium have reduced concentration, orbecause the neutron production is reduced from a lower concentration ofneutron-production target atoms.

Process 350 may begin at step 351, which includes forming a testdrillhole from a terranean surface through first and second rock layers(i.e., below a terranean surface). For example, test drillhole 104 maybe formed (e.g., drilled) from the terranean surface 102 to and throughsubterranean formations 108 and 110. In some aspects, the first andsecond rock layers are located in adjacent (e.g., contacting)subterranean formations. In some aspects, the first and second rocklayers may be separated by one or more intervening subterraneanformations. In some aspects, the first and second rock layers maycomprise a single rock type, but in distinct layers.

Process 350 may continue at step 352, which includes collecting a firstwater sample from the first rock layer at a first depth below theterranean surface. In some aspects, the water sample is brine.Alternatively, step 352 could include collecting a fluid sample. In someaspects, steps 351 and 351 may be replaced or avoided by identifying apreviously collected water sample (e.g., liquid brine) from the firstrock layer.

Process 350 may continue at step 353, which includes determining a firstconcentration of a particular radioisotope in the first water sample. Inan example, the particular radioisotope is Cl-36. The concentration ofCl-36 may be directly measured from the first water sample. Inalternative embodiments, the concentration of the Cl-36 in the firstwater sample may be performed with a direct measurement of the neutronflux in the first rock layer. For example, this can be done by running aneutron monitor (e.g., on a wireline, workstring, or other downholeconveyance) into the drillhole and detecting neutrons as a function ofdepth. These neutrons that create the Cl-36 create the measured fluxvalue and the easily measured concentration of ordinary chlorine 35.Then, the concentration of Cl-36 at the first rock layer can bedetermined.

In other example embodiments, the radioisotope is I-129 and step 353includes a determination of the concentration of I-129 in the firstwater sample. For example, this isotope is produced naturally in theformation in several ways, but at a depth of, e.g., 1 km the primarymethod is spontaneous fission of trace amounts of uranium 235 found inthe formation. Iodine 129 has a half-life of about 16 million years, andit too reaches a secular equilibrium in which the uranium productionmatches the decay rate plus any flow loss.

In another example embodiment to evaluate the concentration of I-129, aniodine pulse can be evaluated if the evaluated formation is relativelyyoung (e.g., less than 100 million years). This method is based on apulse of iodine that is created when organic matter converts tohydrocarbons. This pulse derives from the very high concentration ofiodine that exists in living matter, and which are released when theorganic matter later produces hydrocarbons. Iodine is highlyconcentrated in oceanic organic matter by a factor of typically 4500,compared to the concentration in the water itself. The release of iodineduring hydrocarbon maturation creates a pulse of iodine with a uniqueI-129/I-127 age signature. Iodine mobilized from such a deep organiclayer can migrate upward through formations. The presence of the iodinepulse and its unique age can be used to constrain the age of the sourceformation for hydrocarbons. Since the I-129 concentration releasedduring hydrocarbon production is generally much higher than backgroundI-129 secular equilibrium concentrations, the movement of this oldiodine pulse (which maintains I-129/I-127 age signature) can bedetermined and tracked upward through the rock layers.

In some example embodiments, the particular radioisotope is helium (He)and step 353 includes determining a concentration of He in the firstwater sample. For example, measurements of He gas as a function of depthcan indicate the isolation of the formations for this very mobileelement. Finding high travel velocity for helium does not necessarilymean that the radioisotopes Cl-36 and I-129 will move quickly throughthe rock, but if a low velocity is found for He, that may be anindication that the rock is very tight; chlorine and iodine will, ingeneral, move at a lower velocity than will helium.

Helium may be created in the formation primarily from the alpha decay ofuranium 238, uranium 235, and/or thorium 232. Since helium is stable, itdoes not decay, and the concentration depends primarily on the leakagerate. The production of helium can be calculated from the density of theuranium and thorium isotopes. Then the measured concentration of heliumin step 353 gives a direct result for the stagnancy of the helium gas.In some instances, helium gas flow is expected to occur at a greatervelocity than chlorine or iodine. Thus, if the flow velocity of thehelium is found to be low, it may be an indicator of a high degree ofisolation for the formation (e.g., fluid isolation from the terraneansurface or groundwater sources).

Process 350 may continue at step 354, which includes collecting a secondwater sample from the second rock layer at a second depth below theterranean surface that is deeper than the first depth. In some aspects,the water sample is brine. Alternatively, step 354 could includecollecting a fluid sample. In some aspects, step 354 may be replaced oravoided by identifying a previously collected water sample (e.g., liquidbrine) from the second rock layer.

Process 350 may continue at step 355, which includes determining asecond concentration of the particular radioisotope in the second watersample. Step 355, in some aspects, may be similar to step 353, but withthe second water sample rather than the first. Thus, subsequent to step355, there are two concentrations determined of the particularradioisotope at two different depths under the terranean surface.

Process 350 may continue at step 356, which includes determining thatthe particular radioisotope in the second water sample is at a secularequilibrium based on a ratio of the first and second concentrations ofthe particular radioisotope. In some aspects, a determination that theparticular isotope is at secular equilibrium includes a determinationthat the ratio is at or close to 1. For example, in some aspects whenthe particular radioisotope is uranium, the secular equilibrium levelcan be calculated from the concentrations of uranium determined in steps353 and 355. Any measured departure from the ratio of concentrationsaway from 1 may be an indication of flow of I-129 in the rock.Measurements of I-129 as a function of depth in the rock can determineif the I-129 is stagnant or whether it has been flowing at a velocityhigh enough to be of concern for human safety (e.g., high enough toreach the terranean surface or groundwater sources). Measurements ofiodine concentration versus depth can be used to determine theconcentration gradient of iodine across a number of geologic formations.The iodine gradient can be then used to estimate the diffusive orconductive flow velocity of iodine through the formations. Slow upwardmovement rates for iodine offer an indication that the rock strata canretain I-129 for long periods of time and may be suitable hostformations for nuclear waste disposal in the hazardous waste repository.

In an example when the particular isotope is Cl-36, the ratio ofconcentrations of Cl-36 may be measured as a function of depth in steps353 and 355. From this, it may be determined whether any variabilityseen is consistent with local (e.g., within one or between two or moreformations) secular equilibrium. For example, if a subterranean layerabove a proposed disposal formation (e.g., the formation into which thehazardous waste repository is formed in or with the directionaldrillhole) has low uranium and thorium content, and also shows a lowerconcentration of Cl-36 that is consistent with local secularequilibrium, then that fact provides strong evidence that Cl-36 is notleaking from the lower disposal layer into this upper layer, whichprovides an indicator of isolation (and thus suitability as arepository). On the other hand, if the observed Cl-36 level is higherthan expected from secular equilibrium, than that is a possibleindicator of leakage from below, indicating that the isolation of thelower layer is compromised by flow (and thus not suitable as arepository).

Process 350 may continue at step 357, which includes determining thatthe second rock layer formation is suitable as a hazardous wasterepository based on the determination that the particular radioisotopeis at secular equilibrium.

Process 350 may continue at step 358, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the second rock layer basedon the determination of step 357. Process 350 may continue at step 359,which includes storing hazardous waste material in the hazardous wastestorage repository. Example methods and processes for steps 358 and 359are described with reference to FIGS. 2A-2B and 4.

In some embodiments, a characteristic of process 350 described here isthat it puts limits on the flow velocity of Cl-36, rather than on thebrine itself. When brine flows, the dissolved chlorine can interact withthe rock and flow at a lower velocity. Thus, example embodiments ofprocess 350 measure the flow of the chlorine (or other radioisotope),not that of the water. In some aspects, the movement of chlorine may beone of the more important considerations for safety of humans. Theradioactive waste, itself, has Cl-36 as one of its most dangerouscomponents; direct measurement of the Cl-36 flow is therefore morerelevant than a measure of the water flow. In this considerations,embodiments of the present disclosure are closely tied to a potentialrequirement for safety, because it is the time that Cl-36 takes to reachthe terranean surface and/or groundwater sources (e.g., potable water orotherwise) that has the greatest impact on public security. It isimportant that the travel velocity of Cl-36 is sufficiently slow thatmuch of the radioactive material will have decayed (with its half-lifeof 300,000 years) prior to reaching the terranean surface or groundwatersources.

In some aspects, the selected subterranean zone (e.g., second rocklayer) may include a “cap layer” at a depth between the terraneansurface and a subterranean formation in which the hazardous materialrepository is located. In some aspects, the cap layer may prevent all orsubstantially all fluid flow therethrough. For instance, if there is acap layer at which upward mobile helium is stopped, then that too can betaken as an indicator that disposal at a greater depth offers secureisolation (e.g., preventing radioactive waste from flowing from thehazardous material repository to the terranean surface). Since heliumtends to be more mobile in formations than Cl-36 or I-129 or otherlong-lived radioisotopes, the presence of the cap layer may providegeologic isolation even if the second rock layer used for the repositoryincludes upward flowpaths for leaked radioisotopes.

The present disclosure also describes a process for determining thesuitability of a subterranean formation as a hazardous waste repositorybased on a measurement of a tracer fluid that was added to a drillingfluid used to form a wellbore, such as a test drillhole from which asubterranean fluid sample is collected. For example, turning to FIG. 3F,this figure illustrates an example process 360 for determining thesuitability of a subterranean formation as a hazardous waste repositorybased on a measurement of the tracer fluid from a fluid sample from thesubterranean formation. In some aspects, process 360 may be implementedby or with the system 100, including the fluid testing system 122.

The age of deep brines can be estimated by measuring the levels ofcarbon-14, chlorine-36, iodine-131, and/or other radioactive isotopesthat are produced on the surface from cosmic rays. If the levels of oneor more of these radioisotopes is lower at depth than it is for surface,then it may be concluded that surface water has not flowed downward tothe rock formation that holds the brine for a period determined by thehalf-lives of the radioisotopes. “Surface water” in the presentdisclosure means water from a surface body of water or shallower aquiferthat does not have an artificial increase of a radioactive isotopeattributable to the atmospheric nuclear bomb testing that took place inthe 1950s and 1960s.

For example, if the iodine-129 level in the subterranean water is lessthan half of the iodine-129 level observed in surface water, then it maybe concluded that the brine is as least as old as the half-life ofiodine-129, which is 16 million years. This dating may allow theconclusion that there has not been fluid communication between (e.g.,mobile water flowing therebetween) the surface water and subterraneanwater for 16 million years, thus establishing the rock formation thatholds the subterranean water as a suitable hazardous waste repository.

Subterranean water may suffer the risk of contamination. For example,subterranean water taken from deep wells may have been mixed with waterused in the drilling process (e.g., water used in drilling mud). Suchcontamination typically leads to an underestimate of the age, sincewater used in drilling is typically surface water which has higherradioisotope content. Thus, in some aspects, the contamination ofsubterranean water with surface water in drilling mud may lead to afalse determination that a subterranean formation that holds thecontaminated brine is not suitable as a hazardous waste repository.

Process 360 may begin at step 361, which includes forming a testdrillhole from a terranean surface to a subterranean formation. Forexample, test drillhole 104 may be formed (e.g., drilled) from theterranean surface 102 to the subterranean formation 110. Process 360 maycontinue at step 362, which includes collecting a subterranean watersample from the subterranean formation. In some aspects, the watersample is brine. Alternatively, step 362 could include collecting asubterranean fluid sample. In some aspects, steps 361 and 362 may bereplaced or avoided by identifying a previously collected subterraneanfluid sample (e.g., liquid brine).

Process 360 may continue at step 363, which includes determining aconcentration of a tracer fluid in the subterranean water sample. Forexample, a drilling fluid (e.g., drilling mud) that is used in adrilling process for the test drillhole (or other drillhole or wellbore)may be mixed with a tracer, i.e., something that dissolves in thedrilling fluid and does not precipitate or attach itself to the deeprock surfaces in the subterranean formation. Thus, the subterraneanwater sample may comprise a particular amount or concentration of thetracer. Examples of tracers include thiocyanate (often abbreviated SCN)and fluorobenzoic acid (FBA). Several dye tracers may include theproperty of ease of detectability (using a fluorometer), and the factthat very small amounts of dye (e.g., 1 part per billion) can be readilydetected. Three potential dyes are rhodamine, pyranine, andsulforhodamine. Other tracers are possible.

Process 360 may continue at step 364, which includes comparing thedetermined concentration of the tracer fluid against a threshold valueconcentration. For example, a threshold value may be selected that showsthat there is little to no tracer contamination in the subterraneanwater (or fluid) sample.

Process 360 may continue at step 365, which includes determining thatthe subterranean formation comprises a hazardous waste storagerepository based on the determined concentration of the tracer fluidbeing less than the threshold value. For example, if the tracer isabsent or low (e.g., below the predetermined threshold value), then itmay be determined that contamination from surface water that is part ofthe drilling mud is absent or low. In such a determination, thesubterranean formation may be determined as suitable as a hazardouswaste (e.g., nuclear waste) repository.

In some aspects, process 360 may continue directly from step 365 to 366.In alternative aspects, other testing may be performed as part ofprocess 360. For example, in some aspects, further testing, e.g., withan accelerator mass spectrometry (AMS) system or laser-based resonanceionization (which measures the radioisotope Kr-81 of one of the stableisotopes Kr-80, Kr-82, Kr-84, or Kr-86), may commence to determine theconcentration of the radioactive isotope to determine an accurate age ofthe subterranean water sample. If the age is acceptable (therebyindicating that there has not been movement of surface water to thesubterranean formation), then the formation may be confirmed as suitableas a hazardous waste (e.g., nuclear waste) repository.

Process 360 may continue at step 366, which includes creating thehazardous waste storage repository (or at least initiating creation ofthe hazardous waste repository) in or under the subterranean formationbased on the determination of step 365. Process 360 may continue at step367, which includes storing hazardous waste material in the hazardouswaste storage repository. Example methods and processes for steps 366and 367 are described with reference to FIGS. 2A-2B and 4.

FIG. 2A is a schematic illustration of example implementations of ahazardous waste material storage repository, e.g., a subterraneanlocation for the long-term (e.g., tens, hundreds, or thousands of yearsor more) but retrievable safe and secure storage of hazardous wastematerial, during a deposit or retrieval operation according to thepresent disclosure. The hazardous waste material storage repository maybe formed and operated, for example, subsequent to a determination thatone or both of the subterranean formations 108 or 110 are suitable basedon the radioactive isotope testing of the subterranean water asdescribed in the present disclosure.

Turning to FIG. 2A, this figure illustrates an example hazardous wastematerial storage repository system 200 during a deposit (or retrieval,as described below) process, e.g., during deployment of one or morecanisters of hazardous waste material in a subterranean formation. Asillustrated, the hazardous waste material storage repository system 200includes a drillhole 204 formed (e.g., drilled or otherwise) from theterranean surface 102 and through the subterranean layers 106, 108, and110. In some aspects, the drillhole 204 may be the same as testdrillhole 104 shown in FIG. 1. Alternatively, drillhole 204 may be anenlarged (e.g., reamed or re-drilled) version of test drillhole 104.Alternatively, the drillhole 204 may be a separate drillhole formedthrough the subterranean layers 106, 108, and into 110.

The illustrated drillhole 204 is a directional drillhole in this exampleof hazardous waste material storage repository system 200. For instance,the drillhole 204 includes an access drillhole 206 coupled to aradiussed or curved portion 208, which in turn is coupled to storagedrillhole 210. In this example, the storage drillhole 210 is horizontal.Alternatively, curved portion 208 may be eliminated and storagedrillhole 210 may be a vertical drillhole that couples to verticalaccess drillhole 204 to forma continuous, vertical drillhole.Alternatively, the curved portion 208 may differ from a 90-degree changein direction, in which case the storage drillhole 210 might be tilted.

The illustrated drillhole 204, in this example, has a surface casing 220positioned and set around the drillhole 204 from the terranean surface102 into a particular depth in the earth. For example, the surfacecasing 220 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 204 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardouswaste material storage repository system 200, the surface casing 220extends from the terranean surface through a surface layer 106. In someaspects, the surface casing 220 may isolate the drillhole 204 fromsurface water sources, and may also provide a hanging location for othercasing strings to be installed in the drillhole 204.

As illustrated, a production casing 222 is positioned and set within thedrillhole 204 downhole of the surface casing 220. Although termed a“production” casing, in this example, the casing 222 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 222refers to and includes any form of tubular member that is placed in thedrillhole 204 downhole of the surface casing 220. In some examples ofthe hazardous waste material storage repository system 200, theproduction casing 222 may begin at an end of the radiussed portion 108and extend throughout the inclined portion 110. The casing 222 couldalso extend into the radiussed portion 108 and into the vertical portion106.

As shown, cement 230 is positioned (e.g., pumped) around the casings 220and 222 in an annulus between the casings 220 and 222 and the drillhole204. The cement 230, for example, may secure the casings 220 and 222(and any other casings or liners of the drillhole 204) through thesubterranean formations under the terranean surface 102. In someaspects, the cement 230 may be installed along the entire length of thecasings (e.g., casings 220 and 222 and any other casings), or the cement230 could be used along certain portions of the casings if adequate fora particular drillhole 204. In some aspects the cement may be omittedaltogether. The cement 230, if used, can also provide an additionallayer of confinement for the hazardous waste material in canisters 226.

The storage drillhole portion 210 of the drillhole 204 includes astorage area in a distal part of the portion 210 into which hazardouswaste material may be retrievably placed for long-term storage. Forexample, as shown, a work string 224 (e.g., tubing, coiled tubing,wireline, or otherwise) may be extended into the cased drillhole 204 toplace one or more (three shown but there may be more or less) hazardouswaste material canisters 226 into long-term, but in some aspects,retrievable, storage in the portion 210. For example, in theimplementation shown in FIG. 2A, the work string 224 may include adownhole tool 228 that couples to the canister 226, and with each tripinto the drillhole 204, the downhole tool 228 may deposit a particularhazardous waste material canister 226 in the storage drillhole portion210.

The downhole tool 228 may couple to the canister 226 by, in someaspects, a threaded connection or other type of connection, such as alatched connection. In alternative aspects, the downhole tool 228 maycouple to the canister 226 with an interlocking latch, such thatrotation (or linear movement or electric or hydraulic switches) of thedownhole tool 228 may latch to (or unlatch from) the canister 226. Inalternative aspects, the downhole tool 228 may include one or moremagnets (e.g., rare earth magnets, electromagnets, a combinationthereof, or otherwise) which attractingly couple to the canister 226. Insome examples, the canister 226 may also include one or more magnets(e.g., rare earth magnets, electromagnets, a combination thereof, orotherwise) of an opposite polarity as the magnets on the downhole tool228. In some examples, the canister 226 may be made from or include aferrous or other material attractable to the magnets of the downholetool 228. Alternative techniques for moving the canisters 226 may alsobe used.

FIG. 2A also illustrates an example of a retrieval operation ofhazardous waste material in the storage drillhole portion 210 of thedrillhole 204. A retrieval operation may be the opposite of a depositoperation, such that the downhole tool 228 (e.g., a fishing tool) may berun into the drillhole 204, coupled to the last-deposited canister 226(e.g., threadingly, latched, by magnet, or otherwise), and pull thecanister 226 to the terranean surface 102. Multiple retrieval trips maybe made by the downhole tool 228 in order to retrieve multiple canistersfrom the storage drillhole portion 210 of the drillhole 204.

Each canister 226 may enclose hazardous waste material. Such hazardouswaste material, in some examples, may be biological or chemical waste orother biological or chemical hazardous waste material. In some examples,the hazardous waste material may include nuclear material, such as spentnuclear fuel recovered from a nuclear reactor (e.g., commercial power ortest reactor) or defense nuclear material. For example, a gigawattnuclear plant may produce 30 tons of spent nuclear fuel per year. Thedensity of that fuel is typically close to 10 (10 gm/cm³=10 kg/liter),so that the volume for a year of nuclear waste is about 3 m³. Spentnuclear fuel, in the form of nuclear fuel pellets, may be taken from thereactor and not modified. Nuclear fuel pellet are solid, although theycan contain and emit a variety of radioactive gases including tritium(13 year half-life), krypton-85 (10.8 year half-life), and carbondioxide containing C-14 (5730 year half-life).

In some aspects, one or both of the subterranean formations 108 or 110may contain any radioactive output (e.g., gases) therewithin, even ifsuch output escapes the canisters 226. For example, one or both of thesubterranean formations 108 or 110 may be shown to contain radioactiveoutput based on the test results of the subterranean water testing asdescribed with reference to FIGS. 1 and 3.

Other criteria in addition to the subterranean water testing asdescribed herein may be used to determine that the subterraneanformations 108 or 110 contain any radioactive output (e.g., gases)therewithin. For example, one or both of the subterranean formations 108or 110 may be selected based on diffusion times of radioactive outputthrough the formations 108 or 110. For example, a minimum diffusion timeof radioactive output escaping the subterranean formations 108 or 110may be set at, for example, fifty times a half-life for any particularcomponent of the nuclear fuel pellets. Fifty half-lives as a minimumdiffusion time would reduce an amount of radioactive output by a factorof 1×10¹⁵. As another example, setting a minimum diffusion time tothirty half-lives would reduce an amount of radioactive output by afactor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin spent nuclear fuel because of its long half-life of 24,200 years. Forthis isotope, 50 half-lives would be 1.2 million years. Plutonium-239has low solubility in water, is not volatile, and as a solid, itsdiffusion time is exceedingly small (e.g., many millions of years)through a matrix of the rock formation that comprise the illustratedsubterranean formations 108 or 110 (e.g., shale or other formation). Thesubterranean formations 108 or 110, for example comprised of shale, mayoffer the capability to have such isolation times (e.g., millions ofyears) as shown by the geological history of containing gaseoushydrocarbons (e.g., methane and otherwise) for several million years. Incontrast, in conventional nuclear material storage methods, there was adanger that some plutonium might dissolve in a layer that comprisedmobile ground water upon confinement escape.

In some aspects, the drillhole 204 may be formed for the primary purposeof long-term storage of hazardous waste materials. In alternativeaspects, the drillhole 204 may have been previously formed for theprimary purpose of hydrocarbon production (e.g., oil, gas). For example,one or both of subterranean formations 108 or 110 may be a hydrocarbonbearing formation from which hydrocarbons were produced into thedrillhole 204 and to the terranean surface 102. In some aspects, thesubterranean formations 108 or 110 may have been hydraulically fracturedprior to hydrocarbon production. Further in some aspects, the productioncasing 222 may have been perforated prior to hydraulic fracturing. Insuch aspects, the production casing 222 may be patched (e.g., cemented)to repair any holes made from the perforating process prior to a depositoperation of hazardous waste material. In addition, any cracks oropenings in the cement between the casing and the drillhole can also befilled at that time.

For example, in the case of spent nuclear fuel as a hazardous wastematerial, the drillhole may be formed at a particular location, e.g.,near a nuclear power plant, as a new drillhole provided that thelocation also includes an appropriate subterranean formation 108 or 110,such as a shale formation. Alternatively, an existing well that hasalready produced shale gas, or one that was abandoned as “dry,” (e.g.,with sufficiently low organics that the gas in place is too low forcommercial development), may be selected as the drillhole 204. In someaspects, prior hydraulic fracturing of the subterranean formations 108or 110 through the drillhole 204 may make little difference in thehazardous waste material storage capability of the drillhole 204. Butsuch a prior activity may also confirm the ability of one or both of thesubterranean formations 108 or 110 to store gases and other fluids formillions of years. If, therefore, the hazardous waste material or outputof the hazardous waste material (e.g., radioactive gasses or otherwise)were to escape from the canister 226 and enter the fractured formationof the subterranean formations 108 or 110, such fractures may allow thatmaterial to spread relatively rapidly over a distance comparable in sizeto that of the fractures. In some aspects, the drillhole 204 may havebeen drilled for a production of hydrocarbons, but production of suchhydrocarbons had failed, e.g., because one or both of the subterraneanformations 108 or 110 comprised a rock formation (e.g., shale orotherwise) that was too ductile and difficult to fracture forproduction, but was advantageously ductile for the long-term storage ofhazardous waste material.

FIG. 2B is a schematic illustration of an example implementation of thehazardous waste material storage repository 200 during storage andmonitoring operations according to the present disclosure. For example,FIG. 2B illustrates the hazardous waste material storage repository 200in a long-term storage operation. One or more hazardous waste materialcanisters 226 are positioned in the storage drillhole portion 210 of thedrillhole 204. A seal 234 is placed in the drillhole 204 between thelocation of the canisters 226 in the storage drillhole portion 210 andan opening of the access drillhole 206 at the terranean surface 102(e.g., a well head). In this example, the seal 234 is placed at anuphole end of the curved portion 108. Alternatively, the seal 234 may bepositioned at another location within the access drillhole 206, in thecurved portion 208, or even within the storage drillhole portion 210uphole of the canisters 226. In some aspects, the seal 234 may be placedat least deeper than any source of surface water, such as the surfacewater formation 106. In some aspects, the seal 234 may be formedsubstantially along an entire length of the access drillhole 206.

As illustrated, the seal 234 fluidly isolates the volume of the storagedrillhole 110 that stores the canisters 226 from the opening of theaccess drillhole 206 at the terranean surface 102. Thus, any hazardouswaste material (e.g., radioactive material) that does escape thecanisters 226 may be sealed (e.g., such that liquid, gas, or solidhazardous waste material) does not escape the drillhole 104. The seal234, in some aspects, may be a cement plug or other plug, that ispositioned or formed in the drillhole 204. As another example, the seal234 may be formed from one or more inflatable or otherwise expandablepackers positioned in the drillhole 204. As another example, the seal234 may be formed of a combination of rock and bentonite. As anotherexample, the seal 234 may be formed from rock similar in composition tothe rock found in nearby layers, such as clay-rich shale.

Prior to a retrieval operation (e.g., as discussed with reference toFIG. 2A), the seal 234 may be removed. For example, in the case of acement or other permanently set seal 234, the seal 234 may be drilledthrough or otherwise milled away. In the case of semi-permanent orremovable seals, such as packers, the seal 234 may be removed from thedrillhole 204 through a conventional process as is known.

Monitoring operations may be performed during long-term storage of thecanisters 226. For example, in some aspects, it may be advantageous orrequired to monitor one or more variables during long-term storage ofthe hazardous waste material in the canisters 226. In an example, amonitoring system includes one or more sensors placed in the drillhole204 (e.g., within the storage drillhole 210) and communicably coupled toa monitoring control system through a cable (e.g., electrical, optical,hydraulic, or otherwise) or through a non-cable method (e.g. acousticsignals). The sensors may be placed outside of the casings, or evenbuilt into the casings before the casings are installed in the drillhole204. Sensors could also be placed outside the casing.

The sensors may monitor one or more variables, such as, for example,radiation levels, temperature, pressure, presence of oxygen, a presenceof water vapor, a presence of liquid water, acidity, seismic activity,or a combination thereof. Data values related to such variables may betransmitted along the cable to the monitoring control system. Themonitoring control system, in turn, may record the data, determinetrends in the data (e.g., rise of temperature, rise of radioactivelevels), send data to other monitoring locations, such as nationalsecurity or environmental center locations, and may furtherautomatically recommend actions (e.g., retrieval of the canisters 226)based on such data or trends. For example, a rise in temperature orradioactive level in the drillhole 204 above a particular thresholdlevel may trigger a retrieval recommendation, e.g., to ensure that thecanisters 226 are not leaking radioactive material. In some aspects,there may be a one-to-one ratio of sensors to canisters 226. Inalternative aspects, there may be multiple sensors per canister 226, orthere may be fewer.

FIG. 4 is a flowchart that illustrates an example implementation of aprocess 400 for storing hazardous waste material in a subterraneanformation from which water has been tested for a radioactive isotopeconcentration percentage. Process 400 may begin with step 402, whichincludes moving a storage canister through an entry of a drillhole thatextends into a terranean surface. The storage canister encloses ahazardous waste material, such as chemical, biological, or nuclearwaste, or another hazardous waste material. In some aspects, the storagecanister may be positioned in the entry directly from a mode oftransportation (e.g., truck, train, rail, or otherwise) which broughtthe hazardous waste material to the site of the drillhole. In someaspects, a packaging of the hazardous waste material during transport isnot removed for movement of the storage canister into the entry. In someaspects, such transport packaging is only removed as the storagecanister fully enters the drillhole.

Process 400 may continue at step 404, which includes moving the storagecanister through the drillhole that includes a substantially verticalportion, a transition portion, and a substantially horizontal portion.In some aspects, the drillhole is a directional, or slant drillhole. Thestorage canister may be moved through the drillhole in a variety ofmanners. For example, a tool string (e.g., tubular work string) orwireline may include a downhole tool that couples to the storagecanister and moves (e.g., pushes) the storage canister from the entry tothe horizontal portion of the drillhole. As another example, the storagecanister may ride on rails installed in the drillhole, e.g., a caseddrillhole. As yet another example, the storage canister may be movedthrough the drillhole with a drillhole tractor (e.g., motored or poweredtractor). In another example, the tractor could be built as part of thestorage canister. As yet a further example, the storage canister may bemoved through the drillhole with a fluid (e.g., gas or liquid)circulated through the drillhole.

Process 400 may continue at step 406, which includes moving the storagecanister into a storage area located within or below a subterraneanformation which has been determined as suitable as a hazardous wasterepository. For example, the horizontal portion of the drillhole mayinclude or be coupled to the storage area and may be formed through thesubterranean formation. In some aspects, the subterranean formation mayinclude one or more geologic qualities (proven by one or more of theprocesses described herein) that provide for a fluidic seal (e.g., gasand liquid) against the escape of any hazardous waste material beyondthe formation (e.g., vertically or horizontally).

Process 400 may continue at step 408, which includes forming a seal inthe drillhole that isolates the storage portion of the drillhole fromthe entry of the drillhole. For example, once the storage canister ismoved into the storage area (or after all storage canisters are movedinto the storage area), a seal may be formed in the drillhole. The sealmay be a cement plug, an inflatable seal (e.g., packer), a regioncontaining a mixture of rock and bentonite, or other seal or combinationof such seals. In some aspects, the seal is removable so as tofacilitate a subsequent retrieval operation of the storage canister.

Process 400 may continue at step 410, which includes monitoring at leastone variable associated with the storage canister from a sensorpositioned proximate the storage area. The variable may include one ormore of temperature, radioactivity, seismic activity, oxygen, watervapor, acidity, or other variable that indicates a presence of thehazardous waste material (e.g., within the drillhole, outside of thestorage canister, in the rock formation, or otherwise). In some aspects,one or more sensors may be positioned in the drillhole, on or attachedto the storage canister, within a casing installed in the drillhole, orin the rock formation proximate the drillhole. The sensors, in someaspects, may also be installed in a separate drillhole (e.g., anotherhorizontal or vertical drillhole) apart from the storage area.

Process 400 may continue at step 412, which includes recording themonitored variable at the terranean surface. For example, variable datareceived at the one or more sensors may be transmitted (e.g., on aconductor or wirelessly) to a monitoring system at the terraneansurface. The monitoring system may perform a variety of operations. Forexample, the monitoring system may record a history of one or more ofthe monitored variables. The monitoring system may provide trendanalysis in the recorded variable data. As another example, themonitoring system may include one or more threshold limits for each ofthe monitored variables, and provide an indication when such thresholdlimits are exceeded.

Process 400 may continue at step 414, which includes determining whetherthe monitored variable exceeds a threshold value. For example, the oneor more sensors may monitor radioactivity in the drillhole, e.g., anamount of radiation emitted by the hazardous waste material, whether inalpha or beta particles, gamma rays, x-rays, or neutrons. The sensors,for instance, may determine an amount of radioactivity, in units ofmeasure of curie (Ci) and/or becquerel (Bq), rads, grays (Gy), or otherunits of radiation. If the amount of radioactivity does not exceed athreshold value that, for example, would indicate a large leak ofhazardous nuclear material from the storage canister, then the process400 may return to step 410.

If the determination is “yes,” process 400 may continue at step 416,which includes removing the seal from the drillhole. For example, insome aspects, once a threshold value (or values) is exceeded, aretrieval operation may be initiated by removing the seal. Inalternative aspects, exceeding of a threshold value may notautomatically trigger a retrieval operation or removal of the drillholeseal. In some aspects, there may be multiple monitored variables, and a“yes” determination is only made if all monitored variables exceed theirrespective threshold values. Alternatively, a “yes” determination may bemade if at least one monitored variable exceeds its respective thresholdvalue.

Process 400 may continue at step 418, which includes retrieving thestorage canister from the storage area to the terranean surface. Forexample, once the seal is removed (e.g., drilled through or removed tothe terranean surface), the work string may be tripped into thedrillhole to remove the storage canister for inspection, repair, orotherwise. In some aspects, rather than removing the seal from thedrillhole to retrieve the storage canister, other remedial measures maybe taken. For example, if the determination is “yes” in step 414, ratherthan recovering the hazardous waste material, a decision might be madeto improve the seal. This could be done, for example, by injectingbentonite, a cement, or other sealant into the borehole to fill thespace previously filled with gas.

FIG. 5 is a schematic illustration of an example controller 500 (orcontrol system) according to the present disclosure. For example, thecontroller 500 can be used for the operations described previously, forexample as implementing all or some of the steps of any one of processesdescribed in FIGS. 3A-3F and 4.

The controller 500 is intended to include various forms of digitalcomputers, such as printed circuit boards (PCB), processors, digitalcircuitry, or otherwise. Additionally the system can include portablestorage media, such as, Universal Serial Bus (USB) flash drives. Forexample, the USB flash drives may store operating systems and otherapplications. The USB flash drives can include input/output components,such as a wireless transmitter or USB connector that may be insertedinto a USB port of another computing device.

The controller 500 includes a processor 510, a memory 520, a storagedevice 530, and an input/output device 540. Each of the components 510,520, 530, and 540 are interconnected using a system bus 550. Theprocessor 510 is capable of processing instructions for execution withinthe controller 500. The processor may be designed using any of a numberof architectures. For example, the processor 510 may be a CISC (ComplexInstruction Set Computers) processor, a RISC (Reduced Instruction SetComputer) processor, or a MISC (Minimal Instruction Set Computer)processor.

In one implementation, the processor 510 is a single-threaded processor.In another implementation, the processor 510 is a multi-threadedprocessor. The processor 510 is capable of processing instructionsstored in the memory 520 or on the storage device 530 to displaygraphical information for a user interface on the input/output device540.

The memory 520 stores information within the controller 500. In oneimplementation, the memory 520 is a computer-readable medium. In oneimplementation, the memory 520 is a volatile memory unit. In anotherimplementation, the memory 520 is a non-volatile memory unit.

The storage device 530 is capable of providing mass storage for thecontroller 500. In one implementation, the storage device 530 is acomputer-readable medium. In various different implementations, thestorage device 530 may be a floppy disk device, a hard disk device, anoptical disk device, a tape device, flash memory, a solid state device(SSD), or a combination thereof.

The input/output device 540 provides input/output operations for thecontroller 500. In one implementation, the input/output device 540includes a keyboard and/or pointing device. In another implementation,the input/output device 540 includes a display unit for displayinggraphical user interfaces.

The features described can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The apparatus can be implemented in a computerprogram product tangibly embodied in an information carrier, forexample, in a machine-readable storage device for execution by aprogrammable processor; and method steps can be performed by aprogrammable processor executing a program of instructions to performfunctions of the described implementations by operating on input dataand generating output. The described features can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. A computer program is a set of instructionsthat can be used, directly or indirectly, in a computer to perform acertain activity or bring about a certain result. A computer program canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, solid statedrives (SSDs), and flash memory devices; magnetic disks such as internalhard disks and removable disks; magneto-optical disks; and CD-ROM andDVD-ROM disks. The processor and the memory can be supplemented by, orincorporated in, ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) or LED (light-emitting diode) monitorfor displaying information to the user and a keyboard and a pointingdevice such as a mouse or a trackball by which the user can provideinput to the computer. Additionally, such activities can be implementedvia touchscreen flat-panel displays and other appropriate mechanisms.

The features can be implemented in a control system that includes aback-end component, such as a data server, or that includes a middlewarecomponent, such as an application server or an Internet server, or thatincludes a front-end component, such as a client computer having agraphical user interface or an Internet browser, or any combination ofthem. The components of the system can be connected by any form ormedium of digital data communication such as a communication network.Examples of communication networks include a local area network (“LAN”),a wide area network (“WAN”), peer-to-peer networks (having ad-hoc orstatic members), grid computing infrastructures, and the Internet.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

A first example implementation according to the present disclosureincludes a method that includes collecting a first water sample from afirst rock layer at a first depth below a terranean surface; determininga first concentration of a particular radioisotope in the first watersample; collecting a second water sample from a second rock layer at asecond depth below the terranean surface that is deeper than the firstdepth; determining a second concentration of the particular radioisotopein the second water sample; and based on the second concentration beinggreater than the first concentration by a specified percentage,determining that the second rock layer is suitable as a hazardous wastestorage repository.

An aspect combinable with the first example implementation furtherincludes, based on the determination that the second rock layer issuitable as the hazardous waste storage repository, forming an accessdrillhole from the terranean surface toward the second rock layer.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the particular radioisotope comprises atleast one of U-238, U-235, Th-232, K-40, I-129, or Cl-36.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the specified percentage is at least 50percent.

In another aspect combinable with any of the previous aspects of thefirst example implementation, collecting the first water samplecomprises operating a downhole tool in a test drillhole to collect afirst core sample from the first rock layer; retrieving the first coresample to the terranean surface; and removing the first water samplefrom the first core sample.

In another aspect combinable with any of the previous aspects of thefirst example implementation, collecting the second water samplecomprises operating the downhole tool in the test drillhole to collect asecond core sample from the second rock layer; retrieving the secondcore sample to the terranean surface; and removing the second watersample from the second core sample.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes forming the test drillhole fromthe terranean surface to the second rock layer.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the test drillhole comprises a verticaldrillhole.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises a shaleformation.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises animpermeable layer.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises a leakagebarrier defined by a time constant for leakage of a hazardous wastematerial of 10,000 years or more.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises ahydrocarbon or carbon dioxide bearing formation.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes initiating creation of thehazardous waste storage repository in or under the second rock layer.

In another aspect combinable with any of the previous aspects of thefirst example implementation, initiating creation of the hazardous wastestorage repository in or under the second rock layer comprises formingan access drillhole from the terranean surface toward the second rocklayer; and forming a storage drillhole coupled to the access drillholein or under the second rock layer, the storage drillhole comprising ahazardous waste storage area.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the access drillhole comprises a verticaldrillhole.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the access drillhole is the test drillholecomprises a portion of the access drillhole.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the storage drillhole comprises a curvedportion and a horizontal portion.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises athickness proximate the hazardous waste material storage area of atleast about 200 feet.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the second rock layer comprises athickness proximate the hazardous waste material storage area thatinhibits diffusion of a hazardous waste material through the second rocklayer for an amount of time that is based on a half-life of thehazardous waste material.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes installing a casing in theaccess drillhole and the storage drillhole that extends from at orproximate the terranean surface, through the access drillhole and thestorage drillhole, and into the hazardous waste material storage area ofthe storage drillhole.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes cementing the casing to theaccess drillhole and the storage drillhole.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes, subsequent to forming theaccess drillhole, producing hydrocarbon fluid from the second rocklayer, through the access drillhole, and to the terranean surface.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes storing hazardous waste materialin the hazardous waste storage area.

In another aspect combinable with any of the previous aspects of thefirst example implementation, storing hazardous waste material in thehazardous waste storage area comprises moving a storage canister throughan entry of the access drillhole that extends into the terraneansurface, the entry at least proximate the terranean surface, the storagecanister comprising an inner cavity sized to enclose the hazardous wastematerial; moving the storage canister through the access drillhole andinto the storage drillhole; and moving the storage canister through thestorage drillhole to the hazardous waste storage area.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes forming a seal in at least oneof the access drillhole or the storage drillhole that isolates thehazardous waste storage area from the entry of the access drillhole.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the hazardous waste material comprisesspent nuclear fuel.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the storage canister comprises aconnecting portion configured to couple to at least one of a downholetool string or another storage canister.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes monitoring the hazardous wastematerial stored in the hazardous waste material storage area of thestorage drillhole.

In another aspect combinable with any of the previous aspects of thefirst example implementation, monitoring the hazardous waste materialstored in the hazardous waste material storage area of the storagedrillhole comprises removing the seal; and retrieving the storagecanister from the hazardous waste material storage area to the terraneansurface.

In another aspect combinable with any of the previous aspects of thefirst example implementation, monitoring the hazardous waste materialstored in the hazardous waste material storage area of the storagedrillhole comprises monitoring at least one variable associated with thestorage canister from a sensor positioned proximate the hazardous wastematerial storage area; and recording the monitored variable at theterranean surface.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the monitored variable comprises at leastone of radiation level, temperature, pressure, presence of oxygen,presence of water vapor, presence of liquid water, acidity, or seismicactivity.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes, based on the monitored variableexceeding a threshold value, removing the seal; and retrieving thestorage canister from the hazardous waste material storage drillholeportion to the terranean surface.

Another aspect combinable with any of the previous aspects of the firstexample implementation further includes, prior to collecting either ofthe first or second water samples, measuring a plurality of gamma raysoutput from at least one of the first or second rock layers; based onthe measurement, collecting at least one of the first or second watersamples.

A second example implementation according to the present disclosureincludes a method that includes determining a first concentration of aparticular radioisotope in a first water sample from a first rock layerat a first depth below a terranean surface; determining a secondconcentration of the particular radioisotope in a second water samplefrom a second rock layer at a second depth below the terranean surfacethat is deeper than the first depth; determining that the particularradioisotope in the second water sample is at a secular equilibriumbased on a ratio of the first and second concentrations of theparticular radioisotope; and based on the determination that theparticular isotope in the second water sample is at the secularequilibrium, determining that the second rock layer is suitable as ahazardous waste storage repository.

An aspect combinable with the second example implementation furtherincludes, based on the determination that the second rock layer issuitable as the hazardous waste storage repository, forming an accessdrillhole from the terranean surface toward the second rock layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the particular radioisotope comprises atleast one of I-129, He, or Cl-36.

In another aspect combinable with any of the previous aspects of thesecond example implementation, determining the first and secondconcentrations of the particular isotope comprises measuring a firstneutron flux at a first depth of the first rock layer to determine thefirst concentration; and measuring a second neutron flux at a seconddepth of the second rock layer to determine the second concentration.

In another aspect combinable with any of the previous aspects of thesecond example implementation, determining that the particularradioisotope in the second water sample is at the secular equilibriumcomprises comparing the measured first and second neutron fluxes.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes collecting the first watersample from a test drillhole formed into the first rock layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, collecting the first water samplecomprises operating a downhole tool in the test drillhole to collect afirst core sample from the first rock layer; retrieving the first coresample to the terranean surface; and removing the first water samplefrom the first core sample.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes collecting the second watersample from the test drillhole formed into the second rock layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, collecting the second water samplecomprises operating the downhole tool in the test drillhole to collect asecond core sample from the second rock layer; retrieving the secondcore sample to the terranean surface; and removing the second watersample from the second core sample.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes forming the test drillhole fromthe terranean surface to the second rock layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the test drillhole comprises a verticaldrillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises a shaleformation.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises animpermeable layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises a leakagebarrier defined by a time constant for leakage of a hazardous wastematerial of 10,000 years or more.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises ahydrocarbon or carbon dioxide bearing formation.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes initiating creation of thehazardous waste storage repository in or under the second rock layer.

In another aspect combinable with any of the previous aspects of thesecond example implementation, initiating creation of the hazardouswaste storage repository in or under the second rock layer comprisesforming an access drillhole from the terranean surface toward the secondrock layer; and forming a storage drillhole coupled to the accessdrillhole in or under the second rock layer. The storage drillholeincludes a hazardous waste storage area.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the access drillhole comprises a verticaldrillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the access drillhole is the testdrillhole comprises a portion of the access drillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the storage drillhole comprises a curvedportion and a horizontal portion.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises athickness proximate the hazardous waste material storage area of atleast about 200 feet.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the second rock layer comprises athickness proximate the hazardous waste material storage area thatinhibits diffusion of a hazardous waste material through the second rocklayer for an amount of time that is based on a half-life of thehazardous waste material.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes installing a casing in theaccess drillhole and the storage drillhole that extends from at orproximate the terranean surface, through the access drillhole and thestorage drillhole, and into the hazardous waste material storage area ofthe storage drillhole.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes cementing the casing to theaccess drillhole and the storage drillhole.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes, subsequent to forming theaccess drillhole, producing hydrocarbon fluid from the second rocklayer, through the access drillhole, and to the terranean surface.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes storing hazardous waste materialin the hazardous waste storage area.

In another aspect combinable with any of the previous aspects of thesecond example implementation, storing hazardous waste material in thehazardous waste storage area comprises moving a storage canister throughan entry of the access drillhole that extends into the terraneansurface, the entry at least proximate the terranean surface, the storagecanister comprising an inner cavity sized to enclose the hazardous wastematerial; moving the storage canister through the access drillhole andinto the storage drillhole; and moving the storage canister through thestorage drillhole to the hazardous waste storage area.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes forming a seal in at least oneof the access drillhole or the storage drillhole that isolates thehazardous waste storage area from the entry of the access drillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the hazardous waste material comprisesspent nuclear fuel.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the storage canister comprises aconnecting portion configured to couple to at least one of a downholetool string or another storage canister.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes comprising monitoring thehazardous waste material stored in the hazardous waste material storagearea of the storage drillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, monitoring the hazardous waste materialstored in the hazardous waste material storage area of the storagedrillhole comprises removing the seal; and retrieving the storagecanister from the hazardous waste material storage area to the terraneansurface.

In another aspect combinable with any of the previous aspects of thesecond example implementation, monitoring the hazardous waste materialstored in the hazardous waste material storage area of the storagedrillhole comprises monitoring at least one variable associated with thestorage canister from a sensor positioned proximate the hazardous wastematerial storage area; and recording the monitored variable at theterranean surface.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the monitored variable comprises at leastone of radiation level, temperature, pressure, presence of oxygen,presence of water vapor, presence of liquid water, acidity, or seismicactivity.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes, based on the monitored variableexceeding a threshold value, removing the seal; and retrieving thestorage canister from the hazardous waste material storage drillholeportion to the terranean surface.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the secular equilibrium comprises a ratioin which a production rate of the particular isotope equals a decay rateof the particular isotope.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes determining a flow velocity ofthe particular radioisotope between the second and first rock layersbased on the ratio of the first and second concentrations of theparticular radioisotope.

In another aspect combinable with any of the previous aspects of thesecond example implementation, determining the flow velocity of theparticular radioisotope between the second and first rock layerscomprises determining the flow velocity of the particular radioisotopebetween the second and first rock layers independent of a flow velocityof a brine flow between the second and first rock layers that includesthe particular radioisotope.

A third example implementation according to the present disclosureincludes a method including collecting, from a test drillhole formedfrom a terranean surface to a subterranean formation, a subterraneanwater sample from the subterranean formation; determining aconcentration of a tracer fluid in the subterranean water sample, thetracer fluid comprising a fluid mixed into a drilling fluid used in adrilling process to form the test drillhole; comparing the determinedconcentration of the tracer fluid against a threshold valueconcentration; and based on the determined concentration of the tracerfluid being less than the threshold value, determining that thesubterranean formation comprises a hazardous waste storage repository.

An aspect combinable with the third example implementation furtherincludes, subsequent to the determination that the determinedconcentration of the tracer fluid is less than the threshold value,performing a low level radioisotope dating technique.

In another aspect combinable with any of the previous aspects of thethird example implementation, performing the low level radioisotopedating technique comprises determining, with an accelerator massspectrometry (AMS) system, a concentration of a radioactive isotope ofan element in the subterranean water sample relative to a stable isotopeof the element in the subterranean water sample; comparing thedetermined concentration of the radioactive isotope of the element inthe subterranean water sample with a concentration of the radioactiveisotope of the element in a surface water sample relative to the stableisotope of the element in the surface water sample; and based on thedetermined concentration of the radioactive isotope in the subterraneanwater sample being a specified percentage of the concentration of theradioactive isotope in the surface water sample, determining that thesubterranean formation comprises a hazardous waste storage repository.

In another aspect combinable with any of the previous aspects of thethird example implementation, the radioactive isotope is carbon-14 (¹⁴C)and the stable isotope is ¹²C or ¹³C; the radioactive isotope ischlorine-36 (³⁶Cl) and the stable isotope is ³⁵Cl; the radioactiveisotope is iodine-129 (¹²⁹I) and the stable isotope is ¹²⁷I; theradioactive isotope is beryllium-10 (¹⁰Be) and the stable isotope is⁹Be; or the radioactive isotope is aluminum-26 (²⁶Al) and the stableisotope is ²⁷Al.

In another aspect combinable with any of the previous aspects of thethird example implementation, the specified percentage is less than 50percent.

In another aspect combinable with any of the previous aspects of thethird example implementation, performing the low level radioisotopedating technique comprises determining, with a laser-based resonanceionization system, a concentration of a radioactive isotope of anelement in the subterranean water sample relative to a stable isotope ofthe element in the subterranean water sample; comparing the determinedconcentration of the radioactive isotope of the element in thesubterranean water sample with a concentration of the radioactiveisotope of the element in a surface water sample relative to the stableisotope of the element in the surface water sample; and based on thedetermined concentration of the radioactive isotope in the subterraneanwater sample being a specified percentage of the concentration of theradioactive isotope in the surface water sample, determining that thesubterranean formation comprises a hazardous waste storage repository.

In another aspect combinable with any of the previous aspects of thethird example implementation, the radioactive isotope is Kr-81 and thestable isotope is at least one of Kr-80, Kr-82, Kr-84, or Kr-86.

In another aspect combinable with any of the previous aspects of thethird example implementation, the surface water source comprises atleast one of an aquifer or a water source at the terranean surface incontact with the earth's atmosphere.

In another aspect combinable with any of the previous aspects of thethird example implementation, the tracer fluid comprises at least one ofthiocyanate, fluorobenzoic acid, rhodamine, pyranine, or sulforhodamine.

In another aspect combinable with any of the previous aspects of thethird example implementation, the tracer fluid comprises a dye.

In another aspect combinable with any of the previous aspects of thethird example implementation, the tracer fluid comprises a fluid thatcomprises properties of low absorption in rock and ease ofdetectability.

In another aspect combinable with any of the previous aspects of thethird example implementation, the hazardous waste storage repository isconfigured to store nuclear waste.

In another aspect combinable with any of the previous aspects of thethird example implementation, the nuclear waste comprises spent nuclearfuel.

A fourth example implementation according to the present disclosureincludes a system that includes a subterranean water sample collectionapparatus configured to collect, from a test drillhole formed from aterranean surface to a subterranean formation, a subterranean watersample from the subterranean formation; and a subterranean water sampletest apparatus. The subterranean water sample test apparatus isconfigured to determine a concentration of a tracer fluid in thesubterranean water sample, the tracer fluid comprising a fluid mixedinto a drilling fluid used in a drilling process to form the testdrillhole; compare the determined concentration of the tracer fluidagainst a threshold value concentration; and based on the determinedconcentration of the tracer fluid being less than the threshold value,provide an indication that the subterranean formation comprises ahazardous waste storage repository.

An aspect combinable with the fourth example implementation furtherincludes an accelerator mass spectrometry (AMS) system configured tosubsequent to the determination that the determined concentration of thetracer fluid is less than the threshold value, determine a concentrationof a radioactive isotope of an element in the subterranean water samplerelative to a stable isotope of the element in the subterranean watersample; compare the determined concentration of the radioactive isotopeof the element in the subterranean water sample with a concentration ofthe radioactive isotope of the element in a surface water samplerelative to the stable isotope of the element in the surface watersample; and based on the determined concentration of the radioactiveisotope in the subterranean water sample being a specified percentage ofthe concentration of the radioactive isotope in the surface watersample, provide an indication that the subterranean formation comprisesa hazardous waste storage repository.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the radioactive isotope is carbon-14(¹⁴C) and the stable isotope is ¹²C or ¹³C; the radioactive isotope ischlorine-36 (³⁶Cl) and the stable isotope is ³⁵Cl; the radioactiveisotope is iodine-129 (¹²⁹I) and the stable isotope is ¹²⁷I; theradioactive isotope is beryllium-10 (¹⁰Be) and the stable isotope is⁹Be; or the radioactive isotope is aluminum-26 (²⁶Al) and the stableisotope is ²⁷Al.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the specified percentage is less than 50percent.

Another aspect combinable with any of the previous aspects of the fourthexample implementation further includes a laser-based resonanceionization system configured to subsequent to the determination that thedetermined concentration of the tracer fluid is less than the thresholdvalue, determine a concentration of a radioactive isotope of an elementin the subterranean water sample relative to a stable isotope of theelement in the subterranean water sample; compare the determinedconcentration of the radioactive isotope of the element in thesubterranean water sample with a concentration of the radioactiveisotope of the element in a surface water sample relative to the stableisotope of the element in the surface water sample; and based on thedetermined concentration of the radioactive isotope in the subterraneanwater sample being a specified percentage of the concentration of theradioactive isotope in the surface water sample, provide an indicationthat the subterranean formation comprises a hazardous waste storagerepository.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the radioactive isotope is Kr-81 and thestable isotope is at least one of Kr-80, Kr-82, Kr-84, or Kr-86.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the surface water source comprises atleast one of an aquifer or a water source at the terranean surface incontact with the earth's atmosphere.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the tracer fluid comprises at least oneof thiocyanate, fluorobenzoic acid, rhodamine, pyranine, orsulforhodamine.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the tracer fluid comprises a dye.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the tracer fluid comprises a fluid thatcomprises properties of low absorption in rock and ease ofdetectability.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the hazardous waste storage repository isconfigured to store nuclear waste.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the nuclear waste comprises spent nuclearfuel.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, exampleoperations, methods, or processes described herein may include moresteps or fewer steps than those described. Further, the steps in suchexample operations, methods, or processes may be performed in differentsuccessions than that described or illustrated in the figures.Accordingly, other implementations are within the scope of the followingclaims.

What is claimed is:
 1. A method, comprising: determining a neutron fluxof a first isotope in a subterranean formation; calculating, based atleast in part on the determined neutron flux of the first isotope, apredicted production rate of a second isotope in the subterraneanformation; calculating a first ratio of the predicted production rate ofthe second isotope relative to a theoretical production rate of a stableform of the second isotope; measuring respective concentrations of thesecond isotope and the stable form of the second isotope in asubterranean water sample from the subterranean formation; calculating asecond ratio of the measured concentration of the second isotoperelative to the measured concentration of the stable form of the secondisotope; based on a comparison of the first and second ratios,determining that the subterranean formation is suitable as a hazardouswaste repository; and based on the determination that the subterraneanformation is suitable as the hazardous waste storage repository, formingan access drillhole from the terranean surface toward the subterraneanformation.
 2. The method of claim 1, wherein the first isotope comprisesa first half-life, and the second isotope comprises a second half-lifelonger than the first half-life.
 3. The method of claim 1, wherein thefirst isotope comprises Ar-39, Fe-59, Co-60, Ni-63, Kr-85, Ni-63, orC-14.
 4. The method of claim 1, wherein the second isotope comprisesCl-36.
 5. The method of claim 4, wherein the stable form of the secondisotope comprises Cl-35.
 6. The method of claim 1, further comprisingcomparing the first and second ratios to determine that the first andsecond ratios are equal.
 7. The method of claim 1, wherein determiningthe neutron flux of the first isotope in the subterranean formationcomprises determining the neutron flux of the first isotope based on abulk rock chemistry of the subterranean formation.
 8. The method ofclaim 7, wherein the bulk rock chemistry comprises an amount of uraniumor thorium, or both, per unit volume of the subterranean formation. 9.The method of claim 1, further comprising collecting the water samplefrom a drillhole formed into the subterranean formation.
 10. The methodof claim 9, wherein collecting the water sample comprises: operating adownhole tool in the drillhole to collect a core sample from thesubterranean formation; retrieving the core sample to the terraneansurface; and removing the water sample from the core sample.
 11. Themethod of claim 9, further comprising forming the drillhole from theterranean surface to the subterranean formation.
 12. The method of claim9, wherein the drillhole comprises a portion of the access drillhole.13. The method of claim 1, further comprising initiating creation of thehazardous waste repository in or under the subterranean formation. 14.The method of claim 13, wherein initiating creation of the hazardouswaste repository in or under the subterranean formation comprises:forming a storage drillhole coupled to the access drillhole in or underthe subterranean formation, the storage drillhole comprising a hazardouswaste storage area.
 15. The method of claim 14, wherein the storagedrillhole comprises a curved portion and a horizontal portion.
 16. Themethod of claim 15, further comprising storing hazardous waste materialin the hazardous waste storage area.
 17. The method of claim 16, whereinstoring hazardous waste material in the hazardous waste storage areacomprises: moving a storage canister through an entry of the accessdrillhole that extends into the terranean surface, the entry at leastproximate the terranean surface, the storage canister including an innercavity sized to enclose the hazardous waste material; moving the storagecanister through the access drillhole and into the storage drillhole;and moving the storage canister through the storage drillhole to thehazardous waste storage area.
 18. The method of claim 17, furthercomprising forming a seal in at least one of the access drillhole or thestorage drillhole that isolates the hazardous waste storage area fromthe entry of the access drillhole subsequent to moving the storagecanister through the storage drillhole to the hazardous waste storagearea.
 19. The method of claim 1, wherein the access drillhole comprisesa vertical drillhole.
 20. The method of claim 1, further comprising:monitoring at least one variable associated with the storage canister orthe hazardous waste material from a sensor positioned proximate thehazardous waste storage area; based on the monitored variable exceedinga threshold value: removing the seal, and retrieving the storagecanister from the hazardous waste storage area to the terranean surface.